Nhan T Nguyen

Ph.D Minh Ha-Duong

Economic Potential of Renewable Energy in Vietnam's Power Sector

Keywords: integrated resource planning, renewable energy, electricity generation

A bottom-up Integrated Resource Planning model is used to examine the economic potential of renewable energy in Vietnam's power sector. In a baseline scenario without renewables, coal provides 44% of electricity generated from 2010 to 2030. The use of renewables could reduce that figure to 39%, as well as decrease the sector's cumulative emission of CO 2 by 8%, SO 2 by 3%, and NO X by 4%. In addition, renewables could avoid installing 4.4 GW in fossil fuel generating capacity, conserve domestic coal, decrease coal and gases imports, improving energy independence and security. Wind could become costcompetitive assuming high but plausible on fossil fuel prices, if the cost of the technology falls to 900 US$/kW.

Introduction

The transformation to a mostly market-driven economy has led Vietnam to faster economic growth with annual rates of 7-8% during the last decade. The need for electricity services in Vietnam has been increasing in parallel with industrial development, migration of people to cities, and rising living standards. As discussed by [START_REF] Nguyen | Strategies for Promotion of Energy Efficient and Cleaner Technologies in the Power Sector[END_REF], and Khanh, Q. Nguyen (2007b), given the high growth of electricity generation in Vietnam, thermal power generation is likely to increase in the years to come. Coal is expected to be the dominant fuel for electricity generation from 2015 to 2030. As a result, the share of CO 2 emission coming from the power sector in the national CO 2 emission inventory is expected to grow.

Yet increasing the use of fossil fuels to meet growing worldwide demand for electricity, especially in developing countries, goes against the need to prevent dangerous climate change globally, and has detrimental health and environmental effects locally.

There is an extensive literature on how to mitigate emissions by using different combinations of primary energy resources to generate electricity in Asia. [START_REF] Chattopadhyay | System approach to emission reduction from a power system in India[END_REF] analyzed some mitigation options such as switching cleaner fuels (i.e. from coal to natural gas) to reduce CO 2 emissions in India's power sector. [START_REF] Fernando | Integrated Resource Planning with Environmental Costs in Developing Countries[END_REF] developed an integrated resource planning approach considering both supply and demand side options to address the twin problems of environmental degradation and capital costs in developing countries. [START_REF] Benjamin | Optimization methods foe electric utility resource planning[END_REF] examined how environmental concerns, increased competition, and growing uncertainty have changed the needs of utility planners for optimization models and surveyed a range of models for electric utility resource planning that have developed in response to those needs. [START_REF] Swisher | Tools and Methods for Integrated Resource Planning[END_REF] assembled all necessary information addressing tools and methods for integrated resource planning to improve energy efficiency and protect the environment. [START_REF] Shrestha | Environmental and Electricity Planning Implications of Carbon Tax and Technological Constraints in a Developing Country[END_REF] developed the integrated resource planning (IRP) model to address the implications of a carbon tax and technological constraints in a developing country. [START_REF] Shrestha | Supply-and Demand-Side Effects of Carbon Tax in the Power Sector: An Integrated Resource Planning Analysis[END_REF] analyzed alternatives for reduction of greenhouse gases from Indonesian electric power generation by integrating supply-and demand-side options in an electric utility planning considering the effects of carbon tax. [START_REF] Shrestha | Supply and Demand Side effects of Power Sector Planning with CO 2 Mitigation Constraints in a Developing Country[END_REF] examined the implications of CO 2 emission targets in Indonesian power sector using the IRP analysis. [START_REF] Somporn | CO 2 mitigation and power generation implications of clean supplyside and demand-side technologies in Thailand[END_REF] applied the IRP model to examine the effects of both supply and demand side options on the CO 2 mitigation potential from the power sector in Thailand. Subhes C. B., and Dang, N.Q. Thang (2004) examined the cogeneration potential of the sugar industry to meet the increasing electricity demand in Vietnam based on avoided cost ground under the IRP framework, Khanh, Q. Nguyen (2007aNguyen ( , 2007b) ) estimated the potential of wind energy using a geographical information system assisted approach and used the MARKAL, a least cost model, to simulate the impacts of wind power generation and CO 2 emission constraints on the future choice of fuels and technologies in Vietnam's power sector. All of these studies suggest that integrating renewable energy sources in a cost-effective way is a necessary answer to the energy/environment dilemma.

This paper attempts to give new insights about what it is possible to do in terms of generating electricity and reducing carbon emissions in Vietnam. The originality of this study is to explore the potential of all renewable energy sources together for electricity generation in Vietnam. To this end, using the IRP model we analyze the optimized integration of a large array of grid-connected renewable energy technologies, i.e. hydro, geothermal, biomass, wind, solar,…,etc., in the power electric generation system to meet the challenges of soaring electricity demand, growing environmental concerns, energy pricing climax, and energy security over the period 2010-2030.

The next section summarizes the development of Vietnam's power sector from 1995 to 2005 and official projections out to 2030. It also summarizes quantitatively the national potential of renewable energy sources for electricity generation. Section 3 presents the IRP model and the two scenarios to be compared: with and without renewables. Results discussed in section 4 examine the extent to which renewables can substitute for coal in the optimal generation mix through 2030, and the economic and environmental benefits of such substitution. Section 5 presents a sensitivity analysis based on electricity demand forecast scenarios, the availability of fuels/electricity sources, trends of fossil fuel prices, and costs of renewable energy technologies. Some policy implications are then proposed for the electric power-generation expansion plan in Vietnam. Section 6 concludes. The current electricity generation system in Vietnam consists of thermal gas-based power plants (39%), hydropower plants (37%), and thermal coal; this last now accounts for approximately 16% of electricity generation, and it will play an increasingly important role in the medium and long term. Transmission and distribution loss in Vietnam remains high, even if it has been significantly reduced from 22% in 1995 to 12% in 2005. EVN has developed a plan to reduce transmission and distribution losses to less than 8% by 2025 (Electricity of Vietnam, 2006a[START_REF]The Sixth Master Plan on Electric Power Sector Development[END_REF].

The power sector and renewables in

The development of the electric power sector in Vietnam is managed using the Power Development Master Plan, which estimates the need for electricity and plans the overall development of the power sector during a 10-year period, taking into account the subsequent 10-year period. The current Sixth Power Development Master Plan was approved by the Prime Minister in July 2007. It projects that an additional capacity of more than 10,000 MW will be required between 2005 and 2010 to meet the rapidly growing demand for electricity services. Figure 1 Renewable energy potentials are commonly classified in different categories of theoretical, technical, and economic potential. Theoretical potential is defined at the maximum energy that could be exploited in a region considering only thermodynamic constraints. Technical potential is defined by the energy that could be yielded using existing technology, and thus depends on the date of assessment. Economic potential is defined by the energy that could be yielded using economically feasible installations. Infrastructure or technical constraints and economic aspects (costs of alternative competitive energy sources) determine the limits of the economic potential [START_REF] Voivontas | Evaluation of Renewable Energy Potential using a GIS Decision Support System[END_REF]. Table 1 shows that Vietnam has lots of renewable energy sources that are not yet fully exploited.

Hydro energy: Vietnam has 2 400 rivers 10 km or longer. The hydro energy economic potential is estimated at 84 TWh/yr, which is more than the electricity consumption of 46 TWh in 2005.

Hydro pump storage energy: Vietnam's economic potential is over 10,000 MW of hydro pump capacity. These resources are mainly located in the northern and southern areas of the country.

Geothermal energy: With more than 300 hot streams from 30 °C to 148 °C, Vietnam is preliminarily estimated to have 1, 400 MW that could be developed for direct use and producing electricity. In which, 400 MW geothermal capacity could be developed for producing electricity up to 2020.

Biomass energy: Biomass resources that could be used for generating electricity include rice husk, paddy straw, bagasse (sugar cane, coffee husk, and coconut shell), and wood and plant residue with a potential of 1000-1600 MW.

Solar energy: Vietnam lies from 23° to 8° North latitude and has good constant solar sources. In the southern and central areas, solar radiation levels range from 4 to 5.9 kWh/m 2 /day uniformly distributed throughout the year. The solar energy in the north estimated to vary from 2.4 to 5.6 kWh/m 2 /day. In what follows, we examine the economic potential of renewables, assuming that no barriers to the adoption of these renewable energy technologies and that they are used optimally from a lowest-cost perspective. We use the integrated resource planning (IRP) model to compare two optimized plans for expanding electricity generation in Vietnam, one with and the other without renewables. To this end, sensitivity analysis is performed to seek greater energy independence and energy security by integrating diverse energy sources and to response the questions of at what conditions would the renewables become costcompetitive with fossil options in Vietnam.

The Integrated Resource Planning (IRP) model

Most developing countries face the chronic problem of insufficient financial resources for developing their electric power sector because such development requires huge capital investments. Ineffective exploitation and use of limited domestic energy resources usually leads to electric power supply shortages, creating the need to import fuel or electricity, which puts a major drain on foreign exchange reserves. Moreover, climate change and public health problems can further complicate the development of the power sector in these countries. The United Nations Conference on Environment and Development agreed in June 1992 that all countries should adopt necessary adequate programs to restrain increases in greenhouse gas emissions. This restraint is an environmental hindrance to the development of the power sector in many developing countries.

To address these challenges, energy planners use optimization methods for electric utility resource planning, which is the selection of power generation and energy efficiency resources not only to meet the increasing need for electricity services with cost-effective reductions in the use of electricity, but also lessen the impacts associated with electricity generation. [START_REF] Shrestha | The Integrated Resource Planning (IRP) Model, An User's Guide[END_REF] (i) Demand constraint: the total power generation by all power plants (existing and future) and generation avoided by demand-side management options should not be less than the total projected power demand in all periods (blocks)1 , seasons, and years of the planning horizon.

(ii) Plant availability constraint: the power generation of each plant is limited to the capacity and availability of the plant during each period of the day.

(iii) Reliability constraint: the total power generation capacity of all the plants and generation capacity avoided by demand-side management options must not be less than the sum of the peak power demand and the reserve margin in each year of the planning horizon.

(iv) Annual energy constraint: a maximum limit is set on the energy generation at each plant based on its existing capacity, availability, and maintenance schedule.

(v) Hydro energy availability constraint: the total energy output of each hydro plant in each season should not exceed the plant's maximum available quantity of hydro energy.

(vi) Maximum potential capacity constraint: total installed capacity of each type of power plant must not exceed the maximum allowable capacity of that plant type.

(vii) Minimum operation capacity constraint: all selected thermal generating units, depending on their characteristics (off-peak, intermediate, peak plants…,etc.) must be operated and dispatched to generate electricity energy production at a certain minimum business running capacity, at least.

(viii) Fuel or resource availability constraint: energy generation from a plant cannot exceed the maximum available quantity of fuel supply resources.

(ix) External power availability constraint: energy generation imported cannot exceed the maximum available quantity of external power generation resources.

(x) Demand side management constraint: the level of energy-efficient device selected in a year must not exceed the maximum feasible level of such device in the year.

We consider 14 alternative generation technologies and 10 kinds of fuels. Table 2 summarizes the technical, economic, and environmental characteristics of these generation technologies. The renewable energy generation technologies (RET) considered are: small and mini hydro, geothermal, wind turbine, solar grid connected, biomass-based integrated gasification combined cycle, and biomass direct combustion. Table 3 displays the fuel prices used in the model. Price escalation is defined as the total annual rate of increase in a cost, including the effects of both inflation and real escalation. We assumed that the fuel prices could increase 1-2 percent per year. All energy prices based 2005 and their escalation rates were estimated by Institute of Energy, 2006c. These were used to prepare and evaluate nuclear power development plan in Vietnam and are relatively moderate compared to the market levels observed in early 2008. We, however, carried out a sensitivity analysis, using higher levels of energy prices for more precisely estimation of renewables generation potential compared to fossil fuels options.

Through IRP simulation, 2 seasons (rainy and dry) are modeled in a year. The load curve in a day of a season is divided into 24 blocks (1 hour/block). Renewable energies (such as wind, solar, and small hydro) generation is modeled correlatively to its intermittent nature.

In the IRP, plant dispatch is modeled under merit order method and the readiness of generating electricity from renewable plants depends on their energy source availability, i.e. generation of wind/solar technology depended on the available level of wind, sunlight in each block of a day, and that of small hydro depended on the water level in each season. In this study, the economic potential of renewable energy and its implications for the development of electric power generation in Vietnam are analyzed by comparing a model run without renewables against a model run with renewables. The two scenarios will be named "B1" and "B2". The B1 scenario assumes that the power sector in Vietnam for a period of 2010-2030 will not develop any renewable energy sources, except for large hydro and hydro pump storage. The B2 scenario assumes that during the same period, all of the economically possible RETs mentioned above will be considered for sustainable development of electric power generation in Vietnam. In all other respects, the scenarios are identical. Both scenarios assume the adoption of highly energy-efficient thermal technologies such as supercritical and IGCC coal-fired plants, NGCC gas-fired plants and they assume that there are no direct climate change policy interventions. The same average predicted load demand, transmission and distribution losses, and electricity consumed in each period (see Table 4) are applied to both scenarios. Furthermore, both assume that nuclear energy and demand-side management are not used.

To meet the rapid increase in electricity demand forecast for 2010-2030, Vietnamese organizations are considering different economic alternatives for expanding the electricitygeneration system. Fuels considered economically viable for producing electricity are domestic fossil fuel resources and imports, including imported electricity. The availability of domestic fuels supply is based on exploiting estimation scenarios of natural gas and coalmining industries locally. The possibilities for importing fuel or electricity sources have been estimated depending on their availability and national financial resources. The electricity import is mainly from hydro sources via ASEAN power interconnection system projects that have been concurred or negotiating with neighbor countries such as China, Lao, and Cambodia. The success of these projects depends on involved countries' economic development, impacts of international market pricing level, national strategy on bilateral and multilateral cooperation, etc.... The Vietnamese Government agencies, however, have also carried out an overall assessment of the feasibility of importing electricity and its purchasing prices from these projects (Institute of Energy, 2006c). In the model, the electricity imports are simulated by different hydro generation sources with purchasing electricity prices varied from 4.3 to 4.9 $cent/kWh. Table 5 indicates the maximum quantity of domestic fuels supply and imports (fuels/electricity) for baseline scenarios and sensitivity analysis.

The scenarios applied in this study coincide with the baseline forecast and estimates from official Vietnamese Government agencies. These predictions, however, include very few renewables.

Results

Since IRP is a bottom-up optimization model, optimizing over a broader technology portfolio can improve results. Thus, it is no surprise that the B2 run (with renewables) performs better economically than the B1 run (without renewables). More interesting, perhaps, are the quantitative differences between the two scenarios, which are summarized in Tables 6 and7 and discussed in more detail below. We examine in turn the implications for electricity planning, the benefits of using renewables from the perspective of domestic energy security, the cost improvements, and the environmental benefits.

Overall, IRP simulation suggests that 4.4 GW could be obtained from renewable energy sources in a cost-effective manner for the production of electricity in Vietnam. Small hydro and geothermal energy account for 45.5% and 31.8% of this quantity, respectively.

The rest comes from biomass energy (bagasse, rice husk, and paddy straw).

Implications for Electricity Planning: How could the combination of diversified energy sources, including unconventional energy sources, enable the expansion of electric power generation in Vietnam? IRP results indicate how the addition of electricity-generating capacity and the diversification of electricity sources could be technologically achieved during the specified period. Compared to the B1 scenario, the more cost-effective combination of energy resources in the B2 scenario reduces the total amount of electricity generated by conventional thermal plants. The IRP simulation suggests that electricity generation based on coal-fired plants can be reduced from 43.6% to 39%, and gas-fired plants reduced from 32.4% to 32.1%. This change is primarily due to the fact that RETs can compete more effectively against traditional electricity sources in terms of cost. The electricity generation based on oil-fired plants would be kept unchanged in both scenarios B1 and B2. This is because no new oil-fired plants would be cost-effective selected and all the existing ones are continuously operated as reserve or peak generating units in the system to provide with a certain level of electricity generation in both scenarios.

Introducing RETs into the electric power-generation system could help the country to avoid installing 4.4 GW in fossil fuel generating capacity. This is because RETs offer the supplyside option of unit sizes of various capacities with low or zero fuel cost. Both advantages could make RETs more cost-effective than other technologies in responding to variations in demand for electric power at different times.

Renewables based capacity is a good but not perfect substitute for fuel-based capacity. The B2 scenario requires an increase of 232 MW in additional total generation capacity. This extract capacity allows, in the B2 case, to use less the least efficient generating plants. As a result, compounding renewable energy sources with conventional sources increases total average thermal efficiency from 46.7% in the B1 scenario to 47.2% in the B2 scenario. It also decreases the weighted average capacity factor2 from 64.7% in the B1 scenario to 64.4% in the B2 scenario. This shows that generation units that are more energy-efficient can be more effectively utilized and some generation units that are less efficient can be relied upon less or even replaced completely. More geothermal grid-connected generation units with very high thermal energy efficiency could help significantly improve the overall average thermal system efficiency.

The total electricity generation as modeled here, which must be equal to or larger than total demand in equilibrium, is reduced by 19.5 TWh in the B2 scenario compared to the B1 scenario during the same outlook period. This is because in the IRP model all selected thermal generating plants (existing and future) are simulated with different minimum operating capacities depending on their characteristics (technologies, fuels, capacity, and off-peak, intermediate, peak operation modes,…,etc.). This simulation implies that whenever a thermal power plant is constructed, it must be dispatched to generate electricity at a certain minimum business running capacity, at least. On this simple ground, when some generating units on reserve with less efficiency but larger minimum operation capacities in scenario B1 are, in scenario B2, replaced by higher efficient units with smaller minimum operation capacities, the accumulated minimum electricity generation from those on reserve in scenario B2 could be reduced. This results in reduction in the total electricity generation in scenario B2 accordingly, compared to scenario B1 over the whole planning period.

The second question of how the development of electric power generation integrated with such RETs can maintain the same quality level of electricity services compared to the B1 scenario is also examined. As an answer to this question, Table 7 shows an decrease in the weighted average loss of load probability (LOLP)3 from 0.05% in the B1 scenario to 0.01% in the B2 scenario and a significant reduction in total expected energy not served (EENS)4 from 57.7 GWh in the B1 scenario to 8.5 GWh in the B2 scenario during the specified planning period. This is because reserve capacity in the B2 scenario increases, and RET units are simulated with higher availability factors and lower unit forced outage rates.

Implications for Natural Energy Resource Conservation and Energy Security:

Figure 3 and Table 7 suggest that cost-effective renewable sources could substitute to a relatively small extent for domestic coal and natural gas. The country could save approximately 141.4 million tons of domestic coal and 1.53 billion m 3 of domestic gas for producing electricity during the specified period. Furthermore, Figure 4 indicates that the country could slightly reduce its imports: the demand for imported resources in the B2 scenario is only 304 million tons of coal, 75 billion m 3 of natural gas, and 377 TWh; compared to the B2 import demand of 311 million tons of coal, 78 billion m 3 of natural gas, and 385.6 TWh. This corresponds to savings of 1.34 billion US$ in fuel/electricity imports.

These results were obtained assuming that the prices for domestic and imported fossil fuels are moderate and have annual inflation rate relatively slowly compared to the market levels observed in recent years. Prices in 2008, for example, reached levels higher than those assumed in the study by 20-50% for coal and gas, 70-85% higher for fuel oil. The annual inflation rate of fuel prices rose from 2.3% to 3% per year (World Energy Outlook, IEA 2006).

Cost and Pricing implications:

In the IRP model, the electricity price in terms of average incremental cost (AIC) and long run average cost (LRAC) does not play any driven role to compute the optimal solution. These costs are calculated, based on the optimal solution computed for the electricitygeneration capacity expansion plan using the following formulas:

( ) ( ) ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + - ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + - - = ∑ ∑ = = T 2 i i 1 i T 1 i i 1 1 r 1 / E E / r 1 / VC C TC AIC (1) ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = ∑ = T 1 i i i r 1 / E / TC LRAC (2)
where TC = present value of total cost including capital, fuel, operation and maintenance costs; C 1 = present value of capital cost in year 1; VC 1 is the total fuel, operation and maintenance, and demand-side management costs in year 1; E 1 and E i are the electricity generation in year 1 and year i; r = discount rate; and T = planning horizon.

Table 7 suggests that a reduction of 2.6% (1.3 billion US$) in the total discounted cost of electric power-generation plan could be gained by optimizing the inclusion of renewables in the mix. Figure 5 and Table 7 compare the B1 and B2 scenarios in terms of annual fuel and variable cost. Since fuel and variable cost account for 60-75% of the cost of electricity production, the lower fuel and variable costs in the B2 scenario (1.1 billion US$ lower compared to the B1 scenario) could lead to lower production costs. In addition, lower capital and fixed operation and maintenance (O&M) costs in the B2 scenario imply that some RETs in Vietnam, such as small hydro, geothermal, and biomass (except wood) could become cost-effective enough to compete against conventional sources.

The reasons of falling over time (especially during 2020-2030) in capacity and fuels costs as indicated in Figure 5 would be explained as: though the average electricity demand is estimated to increase by 15%-18%, the increasing rate of peak load demand in the period of 2020-2030 is relatively lower than that in previous years. In addition, in the IRP simulation the capacity on reserve is set to decrease gradually from 30% in year 2010 down to 0.25 over 2020-2024 and 0.2 over 2025-2030. These would be leading to reducing the additional capacity installed, i.e. capacity costs for the additions would be falling accordingly, to meet the peak load demand over the last years of the study time frame. Furthermore, over this period of 2020-2030, some of existing less efficient plants, i.e. higher fuels consumptions, would be retired and replaced by more efficient ones with lower fuels consumptions. This would result in falling fuel prices at the later part of the study period.

Integrating RETs in the development of electric power generation could not only help the country reduce the financial effect of increased demand for electricity, but it could also reduce electricity pricing in Vietnam by 0.03 $cent/kWh, in terms of average incremental cost (AIC), for the period of 2010-2030.

Environmental Implications:

The IRP permits analysis of the mitigation potential of renewables with respect to emission of CO 2 and other harmful substances in the power sector in Vietnam.

Table 7 shows that the total cumulative CO 2 emission released in the B2 scenario is significantly reduced by 8% over 3825 million tons emitted in the B1 scenario for the period 2010-2030. This is an average reduction of 15 million tons of CO 2 per year during 2010-2030, which is a big fraction of the estimated 36 million tons of total CO 2 emission emitted from Vietnam's electricity-generation industry in 2006 [START_REF] Nguyen | Strategies for Promotion of Energy Efficient and Cleaner Technologies in the Power Sector[END_REF].

Typically for this kind of bottom-up model, the abatement cost is negative.

In addition to mitigating global emission, the country could also avoid 3% of total SO 2 and 4% of total NO X emissions emitted during the same period. This is an average reduction of 3.9 kt of SO 2 and 11.8 kt of NO X per year during 2010-2030, which compares to the estimated 128.2 kt of total SO 2 and 102 kt of total NO X emitted by the sector in Vietnam in 2006 [START_REF] Nguyen | Strategies for Promotion of Energy Efficient and Cleaner Technologies in the Power Sector[END_REF].

In this study, the optimization procedure did not take into account the environmental or health costs of energy sources. Including these costs would reduce the use of conventional thermal power plants, especially coal-fired plants. At the social optimum, emissions of CO 2 and other harmful substances by the electricity-generating industry in Vietnam would be reduced accordingly.

Sensitivity analysis and discussion

The IRP simulation suggests that some power-generating plants based on renewable technologies such as solar, wind, and biomass wood are still not cost-effective, mainly because of the high cost of these technologies. However, these costs are expected to fall over time due to technological innovation.

In this study, sensitivity analyses referred to as SA1 through SA7 were performed for:

+ quantities of fuel/electricity imported and domestic fuels supply during the period 2020-2030, based on the country's policy and availability of fuel/electricity sources ( The first part of this section determines the answer to the question of at what conditions would the renewables become cost-effective with fossil options in Vietnam, and how much these could be cost-effectively generated. Table 8 shows that all assumed potential of small hydro, geothermal, and biomass technologies, except for wood energy could be economically exploited in a cost-effective manner in the scenarios. Among these RETs,

small hydro has the biggest potential for producing electricity. In the B2 scenario, small hydro and geothermal could cost-effectively generate 146.5 TWh and 124.3 TWh, respectively, during 2010-2030. Biomass energy (other than from wood) could costeffectively generate of the remaining 26.8% of the total 370 TWh generated by RETs during this period. In all scenarios of sensitivity analysis, the total electricity generated by geothermal (1400 MW) and biomass (1000 MW) over 2010-2030 would not exceed that in scenario B2. This is because the maximum potential for these two renewables to generate electricity in a cost-effective manner has been achieved. In contrast to the other RETs examined in this study, wind power, wood energy, mini hydro, and solar energies could not be cost-competitive with conventional energy sources to produce electricity at their technology costs and fossil fuel prices assumed in the B2 scenario. The following discusses what would be required to make them cost-effectively competitive.

The higher prices of fossil fuels in scenarios SA5, SA6, and SA7 could make the additional small hydro capacity potential (300 MW) become competitive in terms of costeffectiveness, and in this case small hydro could generate a maximum of 163 TWh in a cost-effective manner. IRP simulation also suggests that mini hydro energy potential of 100 MW could become cost-effective, and that it could cost-effectively generate 4.7 GWh in the case of high fuel prices. In the combined case of high fuel prices and a forecast of high load demand, mini hydro could generate up to 4.9 GWh.

Neither wind power nor wood biomass could become cost-effective, either in the scenario of predicted high load demand or the scenario of increased fuel prices. However, if the cost of power-generation technology based on wood energy fell to approximately 1500 US$/kW (100 US$/kW lower than in the B2 scenario), it would become cost-competitive in the case of high fuel prices. Table 8 suggests that in the SA7 scenario, wood energy could provide 100 MW and 10.6 TWh effectively during the specified period. This key finding argues for investment in wood plantation-based power-generating plants in the Vietnamese power sector.

In the scenario SA6, wind power is found to be cost-effective if its technology cost falls to approximately 900 US$/kW (100 US$/kW lower than the baseline scenario) in the context of high fuel prices. Wind power capacity of 4622 MW could be installed by 2030, with a total of 240.2 TWh generated during the period 2010-2030. This implies the possibility of integrating wind power into the Vietnamese power sector in the form of Clean Development Mechanism (CDM)-funded projects.

In contrast to the other RETs, power-generation technology based on solar energy was not cost-effective in any of case studies even at a promising technological innovation cost level of 1 300 US$/kW. This simulation implies that solar energy gird-connected technology would be still expensive for cost-competitive with others in Vietnam's power sector for the next 20 years, at least.

The second part of this section focuses on argument for seeking greater energy independence and energy security by integrating diverse energy sources for generating electricity in Vietnam under assumptions of market changes in fuels prices and insufficient fossil fuels supply. The SA8 scenario examines high fossil fuel prices, constraints on imported natural gas, coal fuels and electricity, and no constraint on domestic coal fuel are coherently examined. Scenarios SA9 through SA11 look at different quantitative constraints on domestic coal fuel used for producing electricity.

When additional constraints on imports (gas and electricity), and high fossil prices introduced in SA8, reductions in imports capacities would be substituted by capacity additions of domestic coal, gas, large hydro, and renewables (small and mini hydro). IRP simulation indicates that the total 4 GW capacity potential (equals to 205 TWh in electricity generated over 2010-2030) of small and mini hydro would be dispatched to generate electricity over the specified period in this scenario. Wind power and wood biomass are still not cost-effective dispatched in this SA8 scenario because no constraint on domestic coal supply allows the power generation system to rely on low-cost coal. However, when constraints on domestic coal fuel supply are introduced additionally (SA9 through SA11), the power generation system switches away from domestic coal and electricity import towards large hydro and renewables. Table 8 suggests that both wood biomass and wind power would become cost-effective in the SA9 case, and even more in the SA10 and SA11 case.

In the conditions described under SA11, wind power (20 GW of capacity, or 162 TWh in electricity generated over 2010-2030), along with large hydro (18 GW capacity, or 1 316 TWh) and small/mini hydro (4 GW capacity, or 205 TWh) are used.

All data used in this paper are based on official documentation provided by authorized organizations in Vietnam. It seems to be optimistically estimated somehow. We, however, considered the time frame of the study by 2030, instead of 2025 for making the plan more realistic.

Concluding remarks

Renewable energy sources could have a minor but non negligible part in the national plan to generate electric power in Vietnam. The candidate grid-connected generation technologies include small hydro, mini hydro, geothermal, solar, wind turbine, integrated gasification cycle based on biomass (rice husk, paddy straw, wood residue), and direct combustion technologies based on biomass (bagasse). The study did not consider nuclear energy option in the analysis due to public acceptance problems related to nuclear waste disposal risks, national backward scientific standard, poor technical and technical capability, weak industrial infrastructure and regulation system, lack of human resources and professional specialists, etc. Moreover, DSM, the important option for utility electric planning, was not used in this paper in order to focus on examining the role of renewables as energy supply side option compared to fossil fuel options. However, both these options are considered in another overall research paper.

Our IRP simulations agree with Khanh, Q. Nguyen (2007b) in that regardless of whether RETs are included, the power sector in Vietnam will rely primarily on fossil fuels after 2015, especially on coal. Large quantities of CO 2 will be emitted into the atmosphere from electricity generation. The demand for electricity services over 2010-2030 may exceed the domestic fuels supply sources for generating electricity in Vietnam. The country, thus, need to import electricity energy as soon as 2010, coal and natural gas since 2015 and 2016, respectively.

More precisely, this study finds that without renewables, electricity energy generation from fossil fuels may account for 76.34% of the total production of 7389.6 TWh over the specified period. In this case, coal would account for 43.6% of the total production, while hydro would account for only 18.4%. Fossil fuel sources are the primary contributors to the increasing share of CO 2 emissions due to the power sector. The increase would be from 60.7 million tons in 2010 to 352.3 million tons in 2030, equals to a growth rate of 14% per year that will total 3825.3 million tons for the period 2010-2030.

This paper finds that some of small hydro with good hydrographic condition, geothermal, and biomass (except for wood) plants would cost competitive to fossil fuels options in Vietnam. Others with highly intermittent nature like wind, other small/mini hydro and wood biomass would only be competitive under specified conditions.

At a moderate assumption level of fossil fuels prices, 4.4 GW of the renewable energy capacity potentially available could now become cost-effective for replacing conventional fuel-generating capacities to produce electricity in Vietnam. Of the capacities that could operate cost-effective, small hydro energy accounts for 45.5%, geothermal accounts for 31.8%, and biomass energy (bagasse, rice husk, and paddy straw) accounts for the remaining 22.7%. With the contribution of renewables capacities, the share of electricity generation provided by coal fuel could be reduced by 5%. This would reduce the total cumulative CO 2 , SO 2 , and NO X emissions by 8.2%, 3%, and 4%, respectively during the period 2010-2030.

In terms of energy resource conservation and energy security, using renewables could potentially reduce the use of domestic coal and natural gas for producing electricity by 141.4 million tons and 1.53 billion m 3 , respectively for the period 2010-2030. The country would need to import only 303.8 million tons of coal and 75.5 billion m 3 of natural gas and 377 TWh during 2010-2030, instead of 311.2 million tons of coal, 78.4 billion m 3 of natural gas, and 386 TWh that must be imported over the same period in the scenario of none using renewables.

Renewables are called in the following order. Hydro energy (small and mini) accounts for the largest portion to produce electricity in baseline analysis scenarios as well as in sensitivity analysis scenarios. The electricity production of small and mini hydro energy would be increasingly exploited up to its maximum potential of 4 GW to meet the requirement of energy independence and energy security over the specified period.

At higher level of fuels prices, mini hydro with limited capacity of 100 MW and additional small hydro capacity of 330 MW would be added to cost-effective generation sources.

Wind power and wood biomass enter the grid-connected generation portfolio last, unless large (but plausible) changes in policy, market or technological conditions occur. More specifically, building upon Khanh, Q. Nguyen (2007b), the study suggests that wind power could be cost-competitive at high fossil fuel prices (i.e. levels reached in 2008), if its cost falls to approximately 900 US$/kW or if the fossil fuel supply from both import and domestic sources are strongly constrained (see SA9 through SA11 cases). If the regulator wishes to maintain its option to use wind power, it should already take into account its specificities when planning the grid and the capacity expansions. Likewise, power generation based on wood energy could become cost-effective provided that its cost falls around 1500 US$/kW or if the fossil fuel supply from both import and domestic sources are strongly constrained (see SA9 through SA11 cases).

In contrast to the other RETs, solar grid-connected generation technology never becomes cost-competitive with fossil options in the IRP simulations to 2030. Nevertheless, it keeps a role for providing electricity to people off-grid.

This paper focused on the role of renewables for grid-connected power generation. They may play an even larger role in off-grid electrification in remote areas. Vietnam has more than 70% of population living in these areas, where connecting people to the grid would is expensive, complicated or impossible. Over the last several decades, Vietnamese

Government has extended enormous efforts to bring electricity to everybody. At the end of 2005, 88% of households or 95% of communes had access to electricity. According to the Government's rural electrification plan for the period 2001-2010, the remaining 400 communes containing 2 million households, which are un-reached by grid-extension will be supplied with electricity energy with off-grid connected renewable energy options such as mini and micro hydro, solar photovoltaic, and wind generator in forms of standalone and household-sized (Electricity of Vietnam, 2006a).

To conclude, IRP results suggest that the country's available renewable energy sources could potentially contribute to satisfy the soaring electricity demand, mitigate polluting emissions, and enhance energy independence and security over 2010-2030. To realize these economic potentials, many barriers remain to be lifted:

Financial and Infrastructural barriers: As the national energy portfolio is increasingly dominated by fossil fuels, under the existing regulations the negative externalities of fossil fuels are not incorporated in electricity prices. EVN, the single-buyer of electricity, has no obligation to buy from renewable energy projects at a price reflecting the full social benefits of clean energy sources. Renewable energy costs, thus, are still above conventional energy prices. Furthermore, there is a lack of commercial business and infrastructure to provide renewable electricity-generation equipment and services, and a limited access to finance for customers, businesses and project developers.

Information, Capacity and Technical barriers:

There is insufficient awareness/information on renewable technologies and data on the national potential of renewable energy sources. Institutional and Legislative barriers: Insufficient co-ordinations and multiplicity bodies within the Government authorizations responsible for the deployment of renewable energy usage have been described as a major barrier to the successful adoption of these technologies. Specially, the current policy and regulatory framework for promoting the usage of renewable energy is inadequate to drive its development.

Unless these barriers are removed, "Renewable Energy, a large potential in Vietnam wasted" is likely to remain a popular theme in Vietnamese media. ( 1 ) The economic potential consists of total large, medium, small, and mini hydro energy;

Figures and Tables

( 2 ) This economical potential is assumed to be used entirely for electricity generation, with none used for heating purposes;

( 3 ) This economical potential is assumed as the input potential in the IRP model ( 4) This economical potential of wind energy in Vietnam is estimated with different feed-in tariffs varying from 5 to 8 $cent/kWh. Due to the intermittent nature of wind energy, it is a common technical practice that only 20% of total generation system capacity installed could be realistically integrated by wind capacity before 2020 [START_REF] Holttinen | Design and Operation of Power Systems with Large Amounts of Wind Power, first results of IEA collaboration[END_REF]. Thus, 20 GW of wind capacity, equivalent to 20% of total generation system installed capacity in Vietnam in 2030 is assumed as maximum wind energy feasibly developed for producing electricity over 2010-2030 in the IRP model.

  Vietnam: status and perspectives Electricity generation, transmission and distribution in Vietnam are mostly provided by Electricity of Vietnam (EVN), a state-owned monopoly established in October 1994 under the Ministry of Industry. At the end of 2005, the total electricity-generation capacity was 11,340 MW, of which Electricity of Vietnam facilities accounted for approximately 78%.The remainder was owned by other local and foreign Independent Power Producers.

  maps the distribution of electricity-generation sources in Vietnam planned for 2010.

Figure 2

 2 Figure 2 presents the historical and forecasted need for electricity energy in Vietnam since 1990, when the Vietnamese Government launched a comprehensive reform. This reform has helped to improve people's living conditions and has driven the development of the national economy. Gross domestic production (GDP) in Vietnam has experienced a rapid growth rate of 8.2% per annum during 1991-1995. The strong economic growth is the main reason that electricity demand has increased by 13.5% over the same period. Demand then grew faster, by 14%, over the period 1995-2005, together with economic development. The Sixth Master Plan was formulated based on scenarios for development of different economic sectors and regions and a comparative analysis using the three forecasting methods of multiple regression, elasticity and intensity. According to this Plan, the electricity demand is expected to increase by 15% per annum in the low-demand scenario and by 18% per annum in the high-demand scenario over the period of 2010-2030.

  Wind energy: Vietnam has approximately 513 GW of theoretical capacity. Excluding restrictions on the exploitation of the potential, 120.5 GW of wind power capacity, about 10 times the peak load demand in 2005 is estimated economically feasible for producing electricity.

  presents the integrated resource planning (IRP) model, which was developed in 1998 by the Energy Program of the School of Environment and Resources Development of the Asian Institute of Technology. It uses mixed-integer linear programming (MILP) to compute a lowest-cost electricity-generation capacity expansion plan. The objective function of the IRP model is to compute the least cost combination of generation capacities of different generation sources, the level of end-use electrical appliances to be added (i.e. demand side), and the level of electricity generated by different plants subject to the following constraints:

Furthermore, inadequate investment

  for research and development (R&D) leads to unreliable national estimates of renewable energy sources and their technological development, and makes it difficult for planning programs. Moreover, lacks of access to technology, skilled manpower, training facilities and R&D facilities are so far hindering the promotion of renewable energy technologies in Vietnam.

Figure 1 .Figure 2 .

 12 Figure 1. Distribution of electricity generation sources in Vietnam for the period 2005-2010 Source: Institute of Energy, 2006b

Figure 3 .Figure 4 .

 34 Figure 3. Annual fuel demand of domestic natural gas (billion m 3 ) and coal (million tons) for electricity generation during 2010-2030 in the B1 and B2 scenarios

Table 5

 5

	+ changes in fuel prices based on market levels observed in recent years and World
	Energy Outlook, IEA 2006 (Table 3)
	+ reductions in the renewable energy-generation technologies costs-based
	technological innovation evolution (Erik Ahlgren et al., 2007).

)

+ low/high load demand forecasts, which reflect greater or smaller prospects for economic development during 2020-2030 (Table

4

);

Table 1 :

 1 Assessment of the potential for renewable energies to supply electricity in Vietnam

	Energy resources	Economical potential	Current development in 2005	Future development planned up to 2025 by Vietnamese agencies	Reference sources
	Hydro	84 TWh/yr ( 1 )
	+ Large hydro (>30 MW)	18-20 GW	Approximately 4200 MW,	16.6 GW by 2020	Electricity of Vietnam, 2006a, 2006c; Institute of
	+ Small hydro (<30 MW)	2-4 GW	equivalent to 18 TWh/yr,		Energy, 2006b, 2006c; Nguyen Khac, 2007
					2.5-3.2 GW
	+ Mini hydro (<1 MW)	100 MW	exploited from hydro		Electricity of Vietnam 2006a, 2006c; Institute of
	Hydro pump storage	10.2 GW	Negligible	i	10.2 GW	Energy, 2006b
	Geothermal	1.4 GW ( 2 )	Negligible		300-400 MW by 2020	Hoang, H. Quy, 1998; Hoang and Ho, 2000;
						Institute of Energy 2006b, 2006c
	Wind energy	120.5 GW ( 4 ) Negligible		500 MW	TrueWind, 2001; Khanh 2007a; Institute of
						Energy, 2006b, 2006c
	Solar energy	1 GW ( 3 )	Negligible		2-3 MW	Institute of Energy, 2006b, 2006c
	Rice husk	250 MW	Negligible			Nguyen L.T, and Q.C. Tran 2004; Enerteam
						2001; BCSE, 2005; Institute of Energy, 2006a,
	Paddy straw	550 MW	Negligible			2006b, 2006c
					500 MW
	Bagasse	200 MW	Negligible
	Wood residue	100 MW	Negligible

Table 2 :

 2 Characteristics of selected candidate generation technologies

	Candidate plants	Capital cost ($/kW)	Efficiency (%)	Fixed O&M cost ($/kW month)	Variable O&M cost ($/MWh)	Emission factor (kg CO 2 /MWh)
	Conventional coal	1100	40	2.8	0.15	880
	Supercritical coal	1200	43	2.8	0.15	800
	IGCC coal	1300	45	3.55	0.15	704
	NGCC gas	700	54.63	1.98	0.99	370
	Steam Oil	900	43.57	1.63	1.48	730
	Solar grid connected	5500	100	2.5	0	0
	Wind turbine	1000-1300	100	1.35	0	0
	Geothermal	1700-2000	100	2.38	0	0
	Very large hydro	1120	100	0.54	0	0
	Medium and large hydro	1100 -1500	100	0.76	0	0
	Small and mini hydro	1200 -1600	100	1.5	0	0
	Bagasse direct combustion	850	23	3.58	5	71.64
	Biomass IGCC	1600	38.30	3.75	2.9	71.64
	Wood IGCC	1600	38.30	3.75	2.9	71.64
	Source: Institute of Energy (2006a, 2006 b, 2006c), Electricity of Vietnam (2006a, 2006b)

Table 3 :

 3 Fuel prices (based on 2005) assumed in scenarios and in sensitivity analysis

		Scenario analysis	Sensitivity analysis
	Fuel type	Fuel prices	Escalation rate	Fuel prices	Escalation rate
		($/Gcal)	(%)	($/Gcal)	(%)
	Domestic coal (Anthracite)	5	1.5	7.142	1.5
	Imported coal (Bitumen)	6.15	1	9.23	1
	Imported FO	28.37	2	50.66	2
	Imported DO	30.79	2	56.7	2
	Domestic natural gas	15.87	2	17.46	2
	Imported natural gas	18.25	1.5	23.8	1.5
	Bagasse	0.781	1	0.781	1
	Rice husk	0.71	1	0.71	1
	Paddy Straw	0.625	1	0.625	1
	Wood residue	4.4	1	4.4	1
	Source: Institute of Energy (2005, 2006a, 2006b, 2006c), Electricity of Vietnam (2006a, 2006b), World Energy Outlook,
	IEA 2006.

Table 7 :

 7 Technical, economic, and environmental comparison between the B1 scenario (no RETs) and the B2 scenario (with RETs) for the period 2010-2030. Simulations of the Vietnamese power sector were carried out using the IRP model.

	Point of comparison	Without renewables	With renewables	Avoided (absolute)	Avoided (%)
	1. Electricity utility implications
	Total generation capacity added over 2010-2030 (MW)	81788	82020	-232	-0.28
	Total generation capacity installed up to 2030 (MW)	99833	100065	-232	-0.23
	Total electricity generation over 2010-2030 (TWh)	7389.6	7370.1	19.5	0.26
	Average loss of load probability (%)	0.05	0.01	0.04	83.15
	Average expected energy not served (GWh)	57.69	8.51	49.18	85.25
	Average thermal efficiency (%)	46.72	47.20	-0.48	-1.03
	Average capacity factor (%)	64.69	64.31	0.38	0.59
	2.

Implications for energy resource conservation and energy security

	Domestic fuel consumption over 2010-2030
	-Coal (million tons)	971	830	141.4	14.56
	-Gas (billion m 3 )	296	295	1.53	0.52
	Imported fuel consumption over 2010-2030
	-Coal (million tons)	311	304	7.41	2.38
	-Gas (billion m 3 )	78	75	2.93	3.74
	-Oil (million tons)	5.02	5.02	0.00	0.00
	Imported electricity during 2010-2030 (TWh)	385.6	377	8.6	2.23
	3. Economic Implications
	Fuel and variable O&M cost during 2010-2030 (million $)	26 864	25 748	1,116	4.15
	Total discounted planning cost during 2010-2030 (million $)	48 167	46 905	1,262	2.62
	Average incremental cost AIC ($cent/kWh)	4.01	3.98	0.03	0.75
	Long run average cost LRAC ($cent/kWh)	2.97	2.90	0.07	2.36
	4. Environmental Implications
	Total emissions during 2010-2030
	-CO 2 emission (Mton)	3 825	3 512	313	8.20
	-SO 2 emission (Kton)	2 601	2 520	81	3.13
	-NO X emission (Kton)	6 015	5 767	248	4.12

The daily chronological load curve in the model is divided into several blocks (i.e. time intervals) in order to adequately reflect the effects of variations in power demand over various periods of a day

Capacity factor of a power plant is the ratio of its actual total electricity generation during a period to the maximum potential electricity generation if it had operated at full installed capacity during same period. The weighted average capacity factor is calculated from annual capacity factors with weights being the annual shares in cumulative electricity generation during the entire planning horizon.

Loss of load probability (LOLP) is the proportion of time when the available generation is expected to be unable to meet the system load. Weighted average LOLP is calculated from annual LOLP figures of the system with weights being the annual shares in cumulative electricity generation during the entire planning horizon.

Expected energy not served (EENS) is the expected amount of energy not supplied during a period due to insufficient capacity.

Acknowledgements

We thank Professor Ram M. Shrestha of the Energy Program of Institute of Technology in Bangkok, Thailand for his valuable advice on how to develop this work. All errors and opinions remain ours. This work has been implemented at the Centre International de Recherche sur l'Environnement et le Développement (CIRED, France) under a research grant for a Ph.D. program offered by the Centre National de la Recherche Scientifique (CNRS) of France. The material was first presented at 31 st IAEE International Conference held on 18-20 June 2008 in Istanbul, Turkey and at the 16 th EAERE Annual Conference held on from 25 to 28 June, 2008 in Gothenburg, Sweden.

 2006a, 2006b, 2006c, 2006d), Electricity of Vietnam (2006a)


Ewa Napieralska
email: ewa.napieralskajuszczak@univ-artois.fr

Daniel Roger
email: daniel.roger@univ-artois.fr

Krzysztof Komeza
email: krzysztof.komeza@p.lodz.pl

Design of high power solid-state transformers with grain oriented electrical steel cores

Keywords: Solid-State Transformer, grain oriented wound core, Litz wire

In the paper, the design of the high-power Solid-State Transformers is presented. The transformer components such as the core and coils have been selected to minimize losses at high frequencies and high power. The bridge command is discussed. The core has been selected so that it can work at high temperatures higher than coils. Therefore, coils must be made with a specific thermal design including an insulation layer that reduces the thermal flux between the core and the coils made of Litz wires.

I. INTRODUCTION

With more and more renewable intermittent sources in the electrical grid, the whole system's stability becomes more complex. Smart nodes, able to control bidirectional energy flows in real-time in the grid's critical nodes, can be very useful [START_REF] Kuntze | Secure deployment of smartgrid equipment[END_REF]. Solid-State Transformers (SSTs) can perform this function [START_REF] Vaca-Urbano | Power quality with solid state transformer integrated smart-grids[END_REF]. They are also suitable applications requiring a high power density, such as railway traction systems [START_REF] Huber | Volume/weight/cost comparison of a 1mva 10 kv/400 v solid-state against a conventional low-frequency distribution transformer[END_REF]. The central element of an SST is a transformer operating at a high frequency. For SSTs, as for every electronic power converter, the passive components are smaller for higher operating frequencies, but losses are higher. For designing high-power SSTs (over 100 kVA), the frequency-dependent losses in electronic switches and coils become major issues [START_REF] Cougo | Accurate switching energy estimation of wide bandgap devices used in converters for aircraft applications[END_REF]. The coils must be made with Litz wires to limit the increase of the AC resistances and the copper losses. This problem can be studied using a finite-element method (FEM), but it is very time-consuming due to the Litz wire complex 3D geometry. Beyond numerical calculations, several fast analytical methods are based on separate skin calculations, internal proximity, and external proximity losses [START_REF] Rossmanith | Measurement and characterization of high frequency losses in nonideal litz wires[END_REF][START_REF] Sullivan | Optimal choice for number of strands in a litz-wire transformer winding[END_REF][START_REF] Li | An Analytical Loss Model of Litz-Wire Windings for Transformers Excited by Converters With Winding Configurations Considered[END_REF][START_REF] Ehrlich | Fast Numerical Power Loss Calculation for High-Frequency Litz Wires[END_REF].

The paper proposes an SST design consisting of operating at medium frequencies (few kHz), with a reduced number of switching per period (rectangular voltages). For such operating frequencies, the transformers can be designed with a wound magnetic core made of thin grain-oriented electrical steel (GOES) sheets. A previous study shows that, at the same frequency and peak flux, the wound GOES core has lower eddy current losses for rectangular waveforms than sine ones [START_REF] Ichou | Assessment of a grain oriented wound core transformer for solid state converter[END_REF]. Consequently, this converter simple command yields low switching losses and iron losses as low as possible. Because of the GOES high magnetic permeability, the cores can be made with long average magnetic field lines at acceptable magnetizing currents. Such cores offer large windows for coils. Therefore, the Litz wire's current densities can be lower than for standard designs, which yields lower copper losses.

II. SIMPLE STRUCTURE OF A SOLID-STATE TRANSFORMER

The heart of an SST cell is a high or medium frequency transformer placed between two electronic bridge. This structure, called "dual active bridge" (DAB), is presented in Fig. 1. The filtering capacitances 𝐶 ! and 𝐶 " are supposed to be large enough for keeping 𝑈 ! and 𝑈 " constant on a period. The losses in the first electronic bridge (S11… S14) is the sum of the conduction and switching losses. Noting 𝑉 #$%&' the saturation voltage of an IGBT, 𝑡 ( and 𝑡 ) the rise and fall times of the collector current in an IGBT, <𝑖 ! > the mean value of the input current 𝑖 ! and 𝑓 % the switching frequency, the losses are given by: 𝑃 *+,-./ = 𝑃 012,/ + 𝑃 0133/ 𝑃 012,/ ≈ 2𝑉 #$%&' < 𝑖 ! > 𝑃 0133/ ≈ 𝑈 ! < 𝑖 ! > /𝑡 ( + 𝑡 ) 0𝑓 4

The input power of the first bridge is 𝑃 ! = 𝑈 ! < 𝑖 ! > ; therefore, the relative value of conduction losses is all the less as 𝑈 ! is large. This assertion assumes that 𝑉 #$%&' is constant. The switching losses are proportional to the transmitted power and to the switching frequency 𝑓 % . The expressions are similar for the bridge 2. Consequently, it is important to operate at the lowest possible switching frequency, which can be the same as the operating frequency. Therefore, the voltage 𝑣 / is made of a single pulse per half-period, as shown in figure 2. The angle β∈[0,π] determines the RMS value of 𝑣 / ; the phase lag φ is used to synchronize the two bridges to control the sign and the magnitude of power flow through the SST.

III. TRANSFORMER DESIGN

The volume of a single-phase transformer is proportional to the cross-section of its magnetic core and the window area that the core gives the windings. The core section's product by the coil window area is proportional to the product of the maximum flux density by the operating frequency, which must be as high as possible.

A. COES wound core

Several magnetic materials are available: ferrites, amorphous or nanocrystalline materials, and GOES. Ferrites can operate at very high frequencies (> 20kHz), but at limited flux densities (< 0.4T), their relative permeabilities are in the range of 2000, and they can operate up to 120°C. Amorphous and nanocrystalline materials can operate up to 1.2 T but at lower frequencies than ferrites; their permeabilities are higher than ferrites; the maximum operating temperature is similar. The GOES can operate up to 1.8 T but at frequencies under a few kHz. The relative permeability is very high (36000). They can operate up to 500°C. Ferrites and GOES are available in large quantities at reasonable costs; they have a larger industrial feedback than amorphous or nanocrystalline materials.

Figure 3 compares the losses in a GOES wound core at 2 kHz and the same peak flux densities for square and sine voltages. It shows an increase of losses with the flux density, but losses remain lower for rectangular voltages than for a sine one. Other experiments were carried out at high temperatures (Fig. 4). Losses decrease when the core temperature increases because of the natural increase of the resistivity. With a GOES wound core, which offers large windows for coils, it is possible to add thermal insulation between the core and the coils for getting lower core losses with a core at a higher temperature than coils.

B. Coils made with Litz wires

Because of the skin and proximity effects, the AC resistance of coils increases with frequency. For medium frequency large powers transformers, the coils must be made with Litz wires. The turn arrangement in coils and the Litz wire parameters (strand section, number of strands) is critical for designing the medium frequency high power transformer. The turn arrangement must be made for getting a good voltage distribution along coils of the voltage spikes due to the fastfronted voltages.

IV. CONCLUSION

For designing high-power SSTs, the bridge command must be as simple as possible (rectangular voltages with a single pulse per half-period). The wound GOES core can operate at a temperature higher than coils. Therefore, coils must be made with a specific thermal design, including an insulation layer that reduces the core's thermal flux and the coils made of Litz wires. Simulations and measurements are made on individual elements before building the SST prototype.

Fig. 1 .

 1 Fig.1. Structure of a dual active bridge (DAB) with a central transformer.

Fig. 2 .

 2 Fig. 2. Voltage made of a single pulse per half-period.

Fig. 3

 3 Fig.3 Specific losses in a GOES wound core for a rectangular and a sine voltage (experimental results).

Fig. 4 .

 4 Fig. 4. Core losses variations with the core temperature for rectangular voltages (experimental results).


Robin Gueret

Lydie Pelinski

Till Bousquet

Mathieu Sauthier

Vincent Ferey

Antony Bigot

Visible-light-driven carboxylic amine protocol

Visible light-driven CarboxyLic Amine Protocol (CLAP) for the synthesis of 2-substituted piperazines in batch and flow conditions Robin Gueret, Lydie Pelinski, Till Bousquet, * Mathieu Sauthier, Vincent Ferey, Antony Bigot *

Supporting Information Placeholder

[Paste publication-size TOC graphic here] ABSTRACT: Piperazines are privileged scaffolds in medicinal chemistry. Disclosed herein is a visible-light-promoted decarboxylative annulation protocol between a glycine-based diamine and various aldehydes to access 2-aryl, 2-heteroaryl as well as 2-alkyl piperazines. Iridium-based complex [Ir(ppy)2(dtbpy)]PF6 or carbazolyl dicyanobenzene 4CzIPN were found to be the photocatalysts of choice to perform efficiently the transformation under mild conditions whether in batch or in continuous mode.

The piperazine skeleton is the third most common nitrogen heterocyclic core encountered in approved pharmaceuticals (Figure 1). [START_REF] Vitaku | Analysis of the Structural Diversity, Substitution Patterns, and Frequency of Nitrogen Heterocycles among U.S. FDA Approved Pharmaceuticals[END_REF] The construction of this saturated 6-membered ring scaffold has thus been extensively investigated. 2 However, efficient synthetic pathways to access α-substituted piperazines are somewhat limited. Available approaches include the di-or monoketopiperazine reduction, the Mitsunobu transformation, 3 hydroamination, 4 condensation of diamines on diols 5 , α-bromo ester or epoxide derivatives, [START_REF] Vidal-Albalat | Nitroepoxides as Versatile Precursors to 1,4-Diamino Heterocycles[END_REF] lithiation/trapping of N-Boc piperazines, [START_REF] Berkheij | Synthesis of 2-substituted piperazines via direct α-lithiation[END_REF] and photoredox catalysis. 8 In 2013, a breakthrough was achieved by Bode's group when first developing the SnAP (Tin Amine Protocole) 9 shortly followed by the photocatalytic SLAP (SiLicon Amine Protocole) 10 variant (Scheme 1). These straightforward strategies allow access to a wide variety of saturated N-heterocycles including piperazines, morpholines, thiomorpholines, oxazepines and diazepines. For the preparation of piperazines, the SnAP reagents are toxic diaminostannane compounds thus the development by Bode's group of a safer, tin-free SLAP protocole making use of diaminosilyl reagents. In both protocols, these key reagents react either with aldehydes or ketones to generate the corresponding aldimines/ketimines, followed by carbon-centered radical generation. This nucleophilic radical then adds to the aldimine/ketimine group in a 6-endo trig mode, to generate the 6-membered ring and the nitrogen-centered aminyl radical, the reduction of which generates the nitrogen-anion that is finally protonated by the solvent (see Scheme 1). Very recently, based on the same photoinitiated concept, an alternative strategy was described from amino-dihydropyridine (DHP) reagents. Indeed, similarly to alkyl silicon reagents, it was previously found that 4-alkyl Hantzsch ester moiety can lead to the corresponding alkyl radical by oxidative single electron transfer. [START_REF] Nakajima | Visible-Light-Mediated Aromatic Substitution Reactions of Cyanoarenes with 4-Alkyl-1,4-dihydropyridines through Double Carbon-Carbon Bond Cleavage[END_REF] In 2014, the group of MacMillan extended the photocatalyzed oxidative decarboxylation on amino acids. [START_REF] Zuo | Decarboxylative Arylation of α-Amino Acids via Photoredox Catalysis: A One-Step Conversion of Biomass to Drug Pharmacophore[END_REF] This triggered significant interest oriented towards the application of such processes in a wide variety of reactions. 13 Among them, Rueping demonstrated that the resulting α-amino radical could undergo an intermolecular addition onto an imine. [START_REF] Fava | Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals[END_REF] From these studies, we asked ourselves whether a decarboxylative photoredox cyclization process might be envisioned for the construction of 2-substituted piperazines. In this paper, we describe our work towards a photocatalytic approach to the synthesis of such scaffolds from easily available, environmentally benign amino acid-based substrates. In line with the SnAP and SLAP chemistry, we propose to name this new annulation process CarboxyLic Amine Protocol (CLAP).

The viability of our approach was initially investigated on a model reaction between the diamino acid 1 (easily available from natural amino-acid glycine) and 4-fluorobenzaldehyde 2a. The reaction was performed under blue light irradiation in the presence of 1 mol% of the [Ir(ppy)2(dtbpy)]PF6 (Ir1) photocatalyst (Table 1). In a first set of experiments, 2a was reacted in a one to one ratio with diamino carboxylic acid 1. Several inorganic bases (KOH, a Each reaction was performed at room temperature, under blue light irradiation (34W) on a 0.1 mmol scale of 1 in 0.05 M concentration in a MeCN/MeOH (4/1) degassed solution, in the presence of 4.1 eq. of base. b NMR yields using 1,3,5-trimethoxybenzene as internal standard. c Experiment performed in the dark.

Cs2CO3, K2HPO4 and CsF) as well as organic ones (TMG, tetramethylguanidine; DBU, 1,8-diazabicyclo[5,4,0]undec-7-ene) were evaluated (entries 1-6). After 3h of irradiation, we were delighted to observe, in all cases, the formation of the desired cyclized product piperazine 3a. Although acceptable yields were obtained with both organic bases and Cs2CO3, the best result was achieved in the presence of KOH, yielding 90% of 3a (entry 1). Increasing the proportion of aldehyde 2a to 1.4 eq. led to an increase in yield to 95% regardless of whether the irradiation time was maintained at 3h or more interestingly decreased to 30 min (entries 7-8).

Next, the catalytic performance of various photocatalysts, including iridium-or ruthenium-based complexes and the purely organic carbazolyl dicyanobenzene 4CzIPN, was evaluated (entries 11-17). From this survey, Ir1 appears to be the most active and interestingly, remains satisfactory with a catalyst loading as low as 0.1 mol% (entries 11-14). It should be emphasized that the easily accessible organophotocatalyst 4CzIPN showed remarkable effectiveness to perform the reaction, although 5 mol% loading was required (entries 16-17). Blank experiments were conducted either in the absence of photosensitizer (entry 18) or without photoexcitation (entry 19) and this suggested that both are necessary to promote the reaction. 15 From this first successful set of experiments, a plausible mechanism can be already proposed starting from the imine, generated prior to irradiation by condensation between the diamine 1 and the aldehyde 2a (Scheme 2). In a first step, the amino moiety is suspected to be oxidized by the photoexcited iridium catalyst [Ir(ppy)2(dtbpy)]PF6. A consecutive decarboxylation would lead to the α-amino radical, which then would undergo an intramolecular addition onto the imine. From the resulting N-centered radical, Scheme 2. Proposed Mechanism for Ir-based catalyst two pathways can be hypothesized. In accordance with the literature, following the path A, this latter radical might be reduced by the Ir(II) species to give, after protonation by methanol or water, the piperazine. Although this reduction might first seems to be unfavorable (E1/2 red = -1.70 V vs SCE for dialkylaminyl radicals and E1/2 red [Ir(III)/Ir(II)] = -1.51 V vs SCE), Bode has proposed a stabilizing effect of the adjacent substituents, thus rendering the reduction feasible. [START_REF] Hsieh | Silicon amine reagents for the photocatalytic synthesis of piperazines from aldehydes and ketones[END_REF] As we found that the reaction could be performed in the presence of 4CzIPN, which has an even less favorable reduction potential (E1/2 red (4CzIPN/4CzIPN-) = -1.21 V vs SCE), we assume that another mechanism could occur through path B. As such, we envision that the N-centered radical could abstract a hydrogen atom from acetonitrile (bond dissociation energy D298(H-CH2CN) = 405.8 ± 4.2 kJ.mol-1) 16 to afford the piperazine and the cyanomethyl radical • CH2CN. This latter can be readily reduced by the photocatalyst (E1/2 red [ • CH2CN/-CH2CN] = -0.72 V ) 17 thereby closing the catalytic cycle.

With suitable conditions established, i.e. 1 equivalent of amino acid, 1.4 equivalent of aldehyde, 4.1 equivalent of KOH, 1 mol % of Ir1, the scope of the annulation process was examined with a variety of aldehydes (Scheme 3).

Scheme 3. Variation of Substrates a,b

a Each reaction was performed at room temperature, under blue light irradiation (34W) on a 0.1 mmol scale of 1 in 0.05 M concentration in a MeCN/MeOH (4/1) degassed solution. b The values outside bracket are the isolated yields and the values inside brackets are the NMR yields using 1,3,5-trimethoxybenzene as internal standard.

These include diversely substituted benzaldehyde derivatives, heteroaromatics as well as aliphatic aldehydes. For the benzaldehydes, this study revealed that a wide range of substituents, including electron donating or withdrawing ones attached to the benzaldehyde in ortho, meta or para position is well tolerated, furnishing the corresponding piperazines with up to 92% yield. As exceptions, 4-cyanobenzaldehyde led to a lower, albeit satisfactory, yield of 70% while 4-nitrobenzaldehyde failed to give the desired product. The heteroaromatics thiofurfural, furfural and nicotinaldehyde were good performers in this CLAP transformation, furnishing 3k, 3l and 3m in 77, 87 and 80% yields, respectively. Amongst the aliphatic aldehydes, cylohexanecarboxaldehyde and propanal yielded the corresponding annulated adducts 3n and 3o with 99 and 87%, respectively. Unfortunately, when the annulation process was attempted with trifluoroacetaldehyde ethyl hemiacetal the 2-trifluorométhyl piperazine was not obtained. Similarly, the reactions with ketones failed during the prerequisite ketimine formation. Despite several dehydration conditions tested, the predominant product was the 4-benzylpiperazin-2-one resulting from intramolecular lactamization of the substrate 1, with no traces of the desired ketimine.

Following the development of the above carboxylic amine protocol, we discovered that the transformation proceeds very quickly in batch conditions, the reaction being over in approximately 30 minutes. This observation prompted us to transpose this reaction from batch to continuous mode. Compared to classical batch processes, an increased surface exposed to light and more homogeneous irradiation are among the multiple benefits of continuous flow conditions for light-mediated reactions. [START_REF] Sambiagio | Flow Photochemistry: Shine Some Light on Those Tubes! Trends Chem[END_REF] As a powerful tool in organic synthesis, flow chemistry has now become common in a wide range of chemical industries, including the pharmaceutical sector for drug discovery, development and manufacturing. 19 In the photocatalysis area, a good number of batch transformations have been transposed to continuous flow processes. 20 The batch conditions for the carboxylic amine protocol were not directly transposable to flow conditions due to the presence of solids which could clog the flow device. A precipitate, most probably potassium trifluoroacetate, forms during the course of the reaction when KOH is used. This led us to replace KOH by 1,8-diazabicyclo[5.4.0]undec-7-ene, which does not form a precipitate during the reaction. In addition, we decided to test the flow transformation in presence of the 4CzIPN. This photocatalyst, despite its requirement for higher loading than Ir1 is particularly cost-attractive and, being purely organic, does not contain the pricy and potentially toxic iridium heavy metal, residual traces of which would have to be tightly controlled in the piperazine if the end use included biological testing (Permitted Daily Exposure for Ir is 100 ppm/day for oral route administration and 10 ppm/day for I.V.).

Similarly to the batch procedure, the imine was preformed for 30 min before being mixed into a methanol/acetonitrile (4/1) solution containing the photoinitiator (Table 2). After degassing, the mobile phase was introduced into a Vapourtec® photoreactor at an initial flow rate of 1.5 mL.min -1 within 6.7 min of residence time. This led to the piperazine 3a with 65% isolated yield (entry 1). Gratifyingly, an improved yield of 80% was obtained when the residence time was reduced to only 3 min (entry 3). Finally, a scale-up continuous experiment from 0.2 to 2.5 mmol led to the photo-annulated adduct with 77% isolated yield within approximately 30 min (entry 5).

In summary, we have demonstrated that a straightforward synthesis of 2-aryl, 2-heteroaryl as well as 2-alkyl piperazines is possible through a photoinitiated decarboxylative annulation protocol between a diamine and a large variety of aldehydes. Advantages include easy access to the building block 1 derived from natural amino-acid glycine, the use of purely organic photoredox catalyst as well as the successful transposition of this reaction from batch to flow conditions. These render the newly developed CLAP protocol a powerful alternative to the current existing methods for the synthesis of 2-substituted piperazines.

Figure 1 . 1 .

 11 Figure 1. Exemple of drugs containing piperazine core

Table 1 . Optimization of the piperazine synthesis a

 1

	entry	base	aldehyde 2a (eq.)	photocatalyst (mol %)	irradiation	yield (%) b
					time (h)
	1	KOH	1	Ir1 (1)	3	90
	2	Cs2CO3	1	Ir1 (1)	3	60
	3	K2HPO4	1	Ir1 (1)	3	25
	4	CsF	1	Ir1 (1)	3	38
	5	TMG	1	Ir1 (1)	3	57
	6	DBU	1	Ir1 (1)	3	60
	7	KOH	1.4	Ir1 (1)	3	95
	8	KOH	1.4	Ir1 (1)	0.5	95
	9	KOH	1.4	Ir1 (1)	3	90
	10	KOH	1.4	Ir1 (1)	3	85
	11	KOH	1.4	Ir1 (1)	3	75
	12	KOH	1.4	Ir1 (0.5)	3	90
	13	KOH	1.4	Ir1 (0.1)	3	85
	14	KOH	1.4	Ir2 (1)	3	33
	15	KOH	1.4	Ru(bpy)3Cl2 (1)	3	33
	16	KOH	1.4	4CzIPN (1)	3	70
	17	KOH	1.4	4CzIPN (5)	3	92
	18	KOH	1.4	none	3	0
	19 c	KOH	1,4	Ir1 (1)	3	0

Table 2 . Continuous process optimization

 2 Reaction performed with 2.5 mmol (1.09 g) of diamine.

	entry	flow rate	residence time	yield
		(mL.min -1 )	(min)	(%)
	1	1.5	6.7	65%
	2	2.5	4	76%
	3	3.33	3	80%
	4	5	2	75%
	5	3.33	3	77% a
	a

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Experimental procedures and accompanying analytical data ( 1 H and 13 C NMR, IR, and MS) for all new compounds

AUTHOR INFORMATION

ACKNOWLEDGMENT

Sanofi is gratefully acknowledged for financial support and for a post-doctoral grant (R. G.). CNRS, Chevreul institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche, Région Nord -Pas de Calais and FEDER are acknowledged for supporting and funding partially this work. We also would like to thank Céline Delabre (UCCS) for their technical support and Mr Alexandre Farag (trainee in Sanofi) for initial experiments demonstrating the validity of the described approach. Finally, we would like to thank Dr Andrew Van-Sickle (Vitry Drug Substance, Sanofi) for help in proofreading.