Professor Bengt Källén
email: bengt.kallen@med.lu.se

Emma Nilsson

Petra Otterblad Olausson

Antidepressant use during pregnancy. Comparison of data obtained from a prescription ption register and from antenatal care records

Keywords: Antidepressants, interview data, pharmacoepidemiology, prescription register

Purpose: To compare interview data on drug use during pregnancy with data identifed from a register of prescriptions.

Materials: We compared information from the Swedish Prescribed Drug Register with the Swedish Medical Birth Register on antidepressant use. In order to evaluate the clinical significance of difference in ascertainment with the two methods, the rate of preterm births among singletons and of neonatal symptoms were studied.

Results: During the year before the last menstrual period, 1.5% of the women filled prescriptions for antidepressants each month. Already before the pregnancy was known, the rate of filled prescriptions decreased and reached 0.5% towards end of pregnancy. Twentytwo per cent of first trimester use of antidepressant was unidentified using interview data and prescriptions during 2 nd and 3 rd pregnancy months covered only 55% of actual use. When women who filled prescriptions one or three months before the last menstrual period were included, 17 and 43%, respectively, of women were included who probably did not use the drugs in the first trimester. Prescriptions gave a more complete ascertainment of drug use after the first trimester than data from antenatal care which seemed to over-estimate drug use.

Conclusions: Interview data seem to give the most valid results on early use. When interview data are not available, prescription data could be used but should not include prescriptions given earlier than one month before the last menstrual period. Studies of drug use later in pregnancy are best based on prescription data in absence of interview data.

Introduction

In pharmacoepidemiological studies of drug use during pregnancy, various techniques are used [START_REF] Källén | Methodological issues in the epidemiological study of the teratogenicity of drugs[END_REF]. Some studies are based on cohorts of exposed women, often obtained from Teratology Information Services (e.g., antidepressant studies [START_REF] Chambers | Birth outcomes in pregnant women taking fluoxetine[END_REF][START_REF] Mcelhatton | The outcome of pregnancy in 689 women exposed to therapeutic doses of antidepressants. A collaborative study of the European Network of Teratology Information Services (ENTIS)[END_REF][START_REF] Diav-Citrin | Paroxetine and fluoxetine in pregnancy: a prospective, multicentre, controlled, observational study[END_REF]). Such studies are usually limited in size and have therefore little power to detect anything but very strong effects of the drug. In order to get a larger study material, different approaches have been used. At present, a much used methodology is based on linkage between national or regional registers of prescriptions with registers of delivery outcomes. Such studies of antidepressants have been made for instance in Canada [START_REF] Oberlander | Effect of timing and duration of gestational exposure to serotonin reuptake inhibitor antidepressants: population-based study[END_REF][START_REF] Cole | Paroxetine in the first trimester and the prevalence of congenital malformations[END_REF], Finland [START_REF] Malm | Risks assocaited with selective sertonin reuptake inhibitors in pregnancy[END_REF], and Denmark [START_REF] Wogelius | Maternal use of slective serotonin reuptake inhibitors and risk of congenital malformations[END_REF][START_REF] Pedersen | Selective serotonin reuptake inhibitors in pregnancy and congenital malformations: population based cohort study[END_REF]. Timing of drug use during pregnancy is then made from the date of filling the prescription related to the week of pregnancy. This methodology has so far not been validated. There are two problems involved, the filling of a prescription does not necessarily mean that the patient took the drug, and the dating method may be less exact than thought. Often prescriptions filled during a period before the last menstrual period (LMP) have been used as indicating exposure during the first trimester, sometimes including one, sometimes three months before LMP. This obviously increases the uncertainty about if the women used the drugs during pregnancy. On the other hand, women may well use drugs during pregnancy which they have posessed for a long time.

Two recent studies compared interview data with prescription data. In a study from Norway [START_REF] Espnes | Comparison of recorded medication use in the Medical Birth Registry of Norway with prescribed medicines registered in the Norwegian Prescription Database[END_REF] prescription data were used as "golden standard" and the specificity and positive predictive values were calculated for various drug categories and.the uncertainties in the use of prescription data were disregarded.

A Swedish study [START_REF] Stephansson | Drug use during pregnancy in Swedenassessed by the Prescribed Drug Register and the Medical Birth Register[END_REF] made use of one year's data from the Swedish medical birth register and the prescribed drug register (the same sources as in the present study). There was no clear-cut distinction in the analysis between interview data referring to early pregnancy and antenatal information on drug prescriptions after the first trimester and in the comparison they included prescriptions filled 3 months before pregnancy.. With the beginning on July 1, 1994, information on drug use during pregnancy has been included in the Swedish Medical Birth Register (MBR) [120]. Information on early use is based on interviews performed in early pregnancy and information on later use comes from antenatal care records. In a recent publication, these data were used for a study of antidepresssant use and its effect in pregnancy [113].

From July 1 st , 2005, a prescribed drug register exists in Sweden with individual identification of each patient [142]. This gives an opportunity to compare information from this register with that given by the woman and recorded on antenatal care records. We choose to analyze the use of antidepressants which represent a drug group often used for extended periods but where an uncertainty on the safety during pregnancy exists.

Material and Methods

Two sources of information was used. One was the prescribed drug register [142] which contains information on all filled prescriptions in Sweden since July 1 st , 2005 with, among other things, patient identification, date of dispensing, and ATC (Anatomical, Therapeutic, Chemical) code.

The second source was the Medical Birth Register (MBR) [120]. This register was started in 1973 and contains information on antenatal care, delivery, and neonatal conditions, since 1982 based on copies of the original medical records. Information on drug use was included from July 1, 1994 [153]. Nearly all women attend the antenatal care system and the first visit usually takes place before the end of the first trimester, the majority between weeks 10 and 12. At this visit, the woman is interviewed by a midwife, among other things on which drugs she has taken since she became pregnant [13]. During the remaining antenatal care, information on prescribed and recommended drug use is available from copies of antenatal care records. This may be based on an information from the woman that she has began to use the drugs with or without recommendations from the antenatal care. The woman may have got prescriptions from other sources than antenatal care which may not have been recorded.

Drug names are recorded in clear text and are later transferred to ATC codes in a semiautomatic process. Timing in pregnacy weeks of the filling of a prescription is based on the estimated last menstrual period date (LMP), calculated from the date of birth and the estimated pregnancy duration in days. The majority of estimates on pregnancy duration was based on second trimester sonography. Information on gestational duration was missing in 200 women (0.08%).

From MBR, information was obtained on maternal age, parity, smoking habits in early pregnancy, height, and prepregnancy weight (from which body mass index, BMI, was calculated). These variables were used together with year of delivery as co-variates in the calculation of adjusted odds ratios (OR) with 95% confidence intervals (95% CI) for preterm birth in singletons and for some neonatal conditions. The latter consisted of respiratory problems (ICD-10 codes P22-P28), jaundice (P59), hypoglycaemia (P70.4-P70.9), and low Apgar score (<7 at 5 minutes). Estimates of the ORs were made using Mantel-Haenszel procedure and those of the 95% CIs with Miettinen´s method.

The mean gestational week at the first antenatal care center was 12.1 (SD =5.7), the median 10 weaks. The mean number of antenatal care visits after the first visit was 9.0 (SD 4.0). Linkage between the prescribed drugs register and MBR was made with the use of the unique personal identification number which everyone born in Sweden or immigrating to Sweden gets and which is widely used in society and in all health care. Ethics: The study was performed within the responsibilities of the National Board of Health and Welfare and therefore no ethical approval from outside ethical committees was needed.

Results

Pattern of antidepressant prescription in relation to pregnancy

During the period July 1, 2006 to December 31, 2008 we identified 23 151 women (28 039 deliveries) who had a delivery registered in MBR with a known pregnancy duration and had received a prescription for an antidepressant between July 1 st , 2005 and December 31, 2008.

The number of women "at risk" for each month was calculated from MBR, indicating how many women were included pregnant in that month of pregnancy.. Figure 1 shows the monthly percentage of women who had a prescription for an antidepressant during one year before LMP (-12 to-1) and during pregnancy (1-10). The monthly rate was thus about 1.5% before LMPas prescriptions usually represent 3 months use, this would mean that about 4.5% of all pregnant women used antidepressants before LMP. After LMP, the rate started to decrease already during the first monthwhen the woman did not know that she was pregnantand continued to decrease to about one third of the pre-LMP monthly use. The strongest decline was seen after the first trimester.

We identifed 6 832 women who had got a prescription for an antidepressant any time during pregnancy. Table 1 shows the distribution per trimester (<12 weeks, 12-24 weeks, >24weeks after LMP). Seventy-seven per cent of the women had such a prescription during the first trimester, 46% during the second trimester, and 40% during the third trimester. In 23% the woman had the first prescription during pregnancy after the first trimester. Among those who had a prescription during the first trimester, 48% had no further prescription during pregnancy.

Prescriptions during the year before LMP occurred in 77% of the women who had got prescriptions during all three trimesters, slightly fewer in women who had prescriptions during the first trimester but not during the second or third trimester, while women who had the first prescriptions during pregnancy in the second and still more evident in the third trimester had less often had a prescription during the year before LMP.

Comparison of data obtained from prescriptions with data reported by the women in early pregnancy

The total number of women reporting use of antidepressants in early pregnancy (mainly 1 st trimester) is 4 488. These women were compared with women who had got a prescription for antidepressants. This comparison was made with three different exposure windows according to the date of prescription (Table 2).

It is likely that the majority of women who got a prescription during months 2-3 of pregnancy used the drug in early pregnancy. This would mean that 22% of these women did not report the drug use or that the midwife did not record it (Table 2). Another explanation is that the woman used the drug after the first visit to the antenatal care, notably if that visit occurred early. Use of MBR thus identified 78% of the possible exposures while use of the prescription register only identified a little more than half of them.

For the period of one month before LMP to the end of the first trimester, 67% were identifed from MBR and 85% from the prescription register. If we accept 22% as a likely estimate of under-registration in MBR, it means that the true number of users during early pregnancy is 1226 more than recorded in MBR but they explain only 56% (1226/2208) of the excess number of women who got prescriptions during this period without being recorded in MBR .

The remaining 44% most likely did not use antidepressants in early pregnancy and 17% of all women identifed from the prescription register by including prescriptions one month before LMP probably did not use the drugs in early pregnancy.

A similar calculation made for the window three months before LMP to the end of first trimester indicates that only 23% of the excess cases in the prescription register are explainable by non-recorded MBR cases and 43% of all women identified from the prescription register probably did not use antidepressants in early pregnancy.

A comparison of the specific drug in the prescription given during the first trimester and the drug name stated by the woman and recorded by the midwife in MBR shows to a large extent complete agreement (97%). Two types of discrepancies were found. In a total of 54 instances, the name in MBR was unspecifiedthe vast majority (51) referred to escitalopram (Cipralex®) or citalopram (Cipramil®). In other instances, the name given in MBR differed from the name in the prescribed drugs register, again the most common combination was escitalopram and citalopram (n=18). The similarities in the two names (and also in the content of the drugs) can explain the discrepancies: the woman may only have recalled that the drug name began with Cipra.

Comparison of prescriptions given after the first trimester with drug use reported in MBR

From the prescription register, 4 294 women were identfied who had got a prescription of an antidepressant more than 84 days after LMP. In the MBR, 3 096 women had a note indicating the use of an antidepressant after the first antenatal care visit. In 2 452 women, both a prescription and a MBR note existed. Thus in 79% of the women who had a MBR recommendation, a prescription was identified -in 21% a prescription after the first trimester was thus not identified. Either the woman had not filled the prescription obtained from the antenatal care or she had resumed using a drug which she had got earlier.

The total number of possible users after the first trimester is thus 4 978 (4 294 + 3 096 -2 412). Among them the prescription register identified 86% and MBR only 62%. In 38% drug use probably occurred without a documentation in the antenatal care records.

Discrepancies between the type of drug in the prescription register and in MBR occurred in 144 cases (6%). In 47 of them, MBR noted an unspecified antidepressant (42 of them referred to escitalopram or citalopram). In 11 cases, the prescription register stated escitalopram and MBR citalopram, in 3 cases the opposite.

Effect on neonatal conditions after maternal use of antidepressants in late pregnancy

Table 3 shows the presence of preterm birth in singletons and of any one among certain neonatal symptoms in three groups of infants: those whose mothers had filled a prescription of an antidepressant after day 84 but had no such drug use noted in MBR after the first antenatal care visit, those had no prescription but MBR information on antidepressant use after the first antenatal visit, and those who had both.

The odds ratio for preterm birth or having a diagnosis is shown in Table 4, comparing the various sources of information on antidepressant use. Infants born of mothers who had prescriptions on antidepressants without a recording in MBR had a significantly higher rate of preterm births and of neonatal complications than infants born of mothers with only information from MBR. The latter had a significantly lower risk for preterm birth than infants born of women who had both a prescription identified and information in MBR on antidepressant use.

Absence of drug recording in MBR could be due to absence of records from the antenatal care after the first visitpossibly because of non-attendance. If women with no antenatal care records after the first visit were removed from the analysis, the OR for preterm birth among infants born of women with prescriptions for antidepressants after the first trimester but no information in MBR about antidepressant use versus infants born of women without such prescriptions but with information on antidepressant use in MBR decreased slightly (OR=1.56, 95% CI 0.96-2.54, based on 1554 infants) and lost statistical significancethe corresponding OR for infants with certain neonatal symptoms also decreased slightly and lost statistical significance (OR=1.34, 95% CI 0.95-1.87, based on 1601 infants).

Discussion

The study has both strengths and weaknesses. The main strength lies in the relatively large number of cases and the possibility of reliable linkage between registers. The main weakness lies in the inexact timing of drug use in MBR. Early use means use before the first antenatal care visit, and if this occurs early, some drugs used during the first trimester may not be included. On the other hand, if the first visit occurs after the first trimester, some late drug use may be mistaken as first trimester use.

The reason to chose antidepressants for analysis was that even though the drugs are often used during extended periods, a concern for possible adverse effects when used during pregnancy can affect the patient´s decision to use the drugs. This can be seen in Figure 1. A decline in the use of antidepressants (as evaluated from the prescription register) is seen already before the woman can know about her pregnancy, a period when pregnancy may be planned and therefore drugs avoided. The use of antidepressants then decreases markedly during the first trimester and for the rest of the pregnancy usage is only about 1/3 of that before pregnancy. This graph shows the filling of prescriptions, not the acutal use of the drugs.

We have information from MBR on the use of drugs in early pregnancy as stated by the woman at the first antenatal visit. Most likely, if the woman stated that she had used an antidepressant she had actually used itbut the timing may be outside the period of organogenesis. Some women will have used the drugs without it being recorded, either because they did not report it or because the midwife did not record it adequately. This phenomenon may vary between different types of drugs, some may be more "sensitive" than others. In order to get an estimate of the amount of non-reported antidepressants, we analyzed women who had got a prescription for antidepressants during the 2 nd and 3 rd month of pregnancy (the main period of organogenesis) and compared them with women who had reported the use of such drugs in early pregnancy. It then appeared that 22% of the women with a prescription had no MBR information on drug use. There may be different reasons for that discrepancy, but the dominating one is probably that the use was not adequately recorded. This means that the unexposed comparison group will contain some individuals who were actually exposed but as they are few they will hardly affect the risk in the unexposed group and therefore not affect the risk ratio estimate.

We then used the estimate of unrecorded drug exposure to estimate how large proportion of women who got prescriptions before LMP used the drugs during early pregnancy. When the analysis included prescriptions filled during one month before LMP, 17% of the women may not have used the drug during early pregnancy, if prescriptions filled during a 3-month period before LMP were included, the corresponding percentage was 43%. Inclusion of unexposed cases in the exposed group will result in a reduction of the estimated risk. This analysis demonstrates the effect of incomplete ascertainment of exposure or of misclassification. If all exposed infants are not identified, little effect on risk estimates are usually obtained. If a misclassification of cases occurs so unexposed cases are regarded as exposed (e.g., a prescription of a drug is recorded but the woman never used it) it will result in a bias towards no effect. When both interview and prescription information is available, interwiew information is preferrable but in absence of interview data, prescription data are acceptable with the understanding that some underestimate of risk may occur. Preferrably prescriptions filled more than one month before LMP should not be included. In the context of the Swedish Medical Birth Register, exposures during early pregnancy should mainly be based on interview data, possibly supplemented with prescriptions filled during 2 nd and 3 rd month of pregnancy.

For exposure during later parts of pregnancy, it is clear that prescription data are more complete than data from antenatal care. Some women who are then regarded as exposed may not be so if she did not take the drug she had bought. In the absence of interview information, the size of this error cannot be estimated. It will bias risk estimates towards null. This phenomenon may explain why women with a recorded statement of the use of an antidepressant but without any identified prescription had a lower odds ratio for neonatal complications than when a prescription was identified, supposing that the effects on the neonatal conditions actually were caused by the drugs.

Concern has been expressed that interview data on drug use could lead to mistakes in drug identification [START_REF] Pedersen | Selective serotonin reuptake inhibitors in pregnancy and congenital malformations: population based cohort study[END_REF]. Our comparison of discrepanices between the actual drug name according to the prescription and the name reported by the woman indicated that this problem is small but could be a specific problem for two drugs with similar propietary names, Cipramil® (citalopram) and Cipralex® (escitalopram).

Finally, it should be stressed that different drug categories may behave differently in this type of analysis. Examples are drugs for chronic diseases, e.g., anticonvulsant use in epilepsy which are usually taken for an extended period of time. On the other hand, anticonvulsants used as mode stabilizers at bipolar disease may behave like the antidepressants studied in the present paper. Other drugs like antibiotics are usually used only during a short period and the inclusion of drugs prescribed even within one month before LMP is probably not relevant for exposure during the organogenetic period.

In conclusion, we find that antidepressant drug exposure during early pregnancy (of interest in studies of teratogenicity) is best ascertained from prospective interview data. If prescription register data are used, inclusion of prescriptions earlier than one month before LMP increases the percentages of probable non-users considerably and should be avoided. For studies of effects of drug use later in pregnancy, the best source of information is the prescription register even though identification of drug use is then probably incomplete.

Fig. 1 :

 1 Fig. 1: Diagram showing percentage of women who get a prescription for an antidepressant related to last menstrual period (LMP)

Table 3 :

 3 Presence of preterm birth or a neonatal diagnosis of respiratory problems, low Apgar score, intraventricular haemorrhage, hypoglycaemia, and/or jaundice according to source of information on maternal use of antidepressant after the first trimester. Data from MBR and from the Prescribed Drug Register

	Prescriptionbed Drug	MBR Preterm	Total a %	Neonatal	Total %
	Register		births		complications
	Yes	No	164 1785 9.2	286 1842 15.5
	Yes	Yes	184 2416 7.6	343 2488 13.0
	No	Yes	34	629 5.4	66	679 9.9

a Singleton infants with known gestational duration

Table 4 :

 4 Odds ratio (OR) with 95% confidence interval (95% CI) comparing groups of infants according to information of maternal use of antidepressant after the first trimester."Prescription" refers to Prescribed Drug Register.Odds ratio (OR) estimated with Mantel-Haenszel technique with adjustment for year of delivery, maternal age, parity, smoking, and body mass index. Calculation of 95% confidence intervals (95% CI) with Miettinen's method.

		Preterm birth	Neonatal
				complications
	Groups compared	OR	95% CI	OR	95% CI
	Prescription and no MBR	1.10 0.84-1.44	1.20 0.97-1.49
	vs. prescription and MBR
	Prescription and no MBR	1.65 1.03-2.66	1.40 1.00-1.96
	vs. no prescription but MBR
	No prescription but MBR	0.64 0.42-0.97	0.85 0.62-1.17
	vs. prescription and MBR

Formatted: Line spacing: Double

Acknowledgement:

The study was supported by a grant from Evy and Gunnar Sandberg Foundation, Lund to BK. The authors declare that they have no conflict of interest


Joshua M Pearce
email: jpearce@clarion.edu

Industrial Symbiosis of Very Large Scale Photovoltaic Manufacturing

Keywords: solar photovoltaic, industrial ecology, industrial symbiosis

In order to stabilize the global climate the world's governments must make significant commitments to drastically reduce global greenhouse gas (GHG) emissions. One of the most promising methods of curbing GHG emissions is a world transition from fossil fuels to renewable sources of energy. Solar photovoltaic (PV) cells offer a technically sustainable solution to the projected enormous future energy demands. This article explores utilizing industrial symbiosis to obtain economies of scale and increased manufacturing efficiencies for solar photovoltaic cells in order for solar electricity to compete economically with fossil fuel-fired electricity. The state of PV manufacturing, the market and the effects of scale on both are reviewed. Government policies necessary to construct a multi-gigaWatt PV factory and complimentary policies to protect existing solar companies are outlined and the technical requirements for a symbiotic industrial system are explored to increase the manufacturing efficiency while improving the environmental impact of PV. The results of the analysis show that an 8-factory industrial symbiotic system can be viewed as a medium-term investment by any government, which will not only obtain direct financial return, but also an improved global environment. The technical concepts and policy limitations to this approach were analyzed and it was found that symbiotic growth will help to mitigate many of the limitations of PV and is likely to catalyze mass manufacturing of PV by transparently demonstrating that large scale PV manufacturing is technically feasible and reaches an enormous untapped market for PV with low costs.

Introduction

The global climate destabilization underway is primarily due to human combustion of fossil fuels for energy and the resultant greenhouse gas (GHG) emissions [START_REF]The Intergovernmental Panel on Climate Change[END_REF][START_REF]The Intergovernmental Panel on Climate Change. Summary for Policymakers, Climate Change 2001: Impacts, Adaptation, and Vulnerability[END_REF][START_REF]Climate Change 2007: The Physical Science Basis Summary for Policymakers[END_REF]. There is a large consensus among scientists that if current trends continue in climate destabilization, the earth will reach a point of no return [START_REF] Hoffert | Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet[END_REF]. The challenge of reducing atmospheric GHG like carbon dioxide (CO 2 ) emissions is significant -with the Kyoto Protocol an entire order of magnitude below those values necessary to stabilize the global climate [START_REF] Hansen | Greenhouse Gas Growth Rates[END_REF]. To obtain a stable climate, GHG emissions would have to be reduced to a point equal to the natural absorption of CO 2 . After this stabilization occurs, the level of natural absorption will gradually fall as the vegetation sink is exhausted so GHG emissions would need to fall to the level of ocean uptake alone. This level is not well quantified, but may demand emissions reductions to 5 Gt CO 2 eq./year (more than 80% below current levels) by the second half of the next century [START_REF] Prentice | The carbon cycle and atmospheric carbon dioxide[END_REF].

One of the most promising methods of curbing continued GHG emissions and concomitant global warming is for the world to transition to renewable sources of energy. Solar photovoltaic (PV) cells, which convert sunlight directly into electricity, offer a technically feasible and environmentally sustainable solution to our enormous future energy needs [START_REF] Pearce | Photovoltaics -A Path to Sustainable Futures[END_REF]. PV fabrication does entail use of energy and with the current energy blend this is equivalent to GHG emissions, but the energy payback time for modern solar cells is a fraction of their total lifetime so they are net CO 2 sinks [START_REF] Pearce | Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells[END_REF]. With existing technologies, readily available materials, and current conversion efficiencies found in manufactured modules, an insignificant fraction of terrestrially available sunlight is needed to power the global society [START_REF] Pearce | Photovoltaics -A Path to Sustainable Futures[END_REF]. Unfortunately, due to the currently small manufacturing facilities, PV has not reached an economy of scale able to compete economically with conventional energy sources and remains a small but growing niche in the energy market. This article explores utilizing industrial symbiosis to obtain economies of scale and increased manufacturing efficiencies for PV cells in order for solar electricity to compete economically with fossil fuel-fired electricity. This set of technical concepts and policies can be viewed as a medium term investment by a government interested in speeding the advance of renewable energy, which will not only see financial return directly, but also an improved global environment, enhanced national energy security, and international favor. First, the state of PV manufacturing, the market and the effects of scale on both will be reviewed. Next, the government policies necessary to construct a multi-GigaWatt (GW = 10 9 W) PV factory and complimentary policies to protect existing solar companies will be discussed. The technical requirements for a symbiotic industrial system will be outlined to increase the manufacturing efficiency while improving the environmental impact. Finally, how this system could act as a catalyst and the limitations to this approach will be analyzed and conclusions will be drawn.

Solar Photovoltaic Manufacturing 2.1 Photovoltaic Production

Although PV electricity production is a technically feasible method of providing for the world's energy needs and mitigating human interference with the climate, the technology will not be more than a boutique energy source until economic costs are driven Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002 down to be directly competitive with fossil fuel-fired electricity. There is intense interest and often government support throughout the world for PV. Although the well-intentioned government programs [START_REF] Ellis | Renewable energy promotion in IEA countries[END_REF] including: government-funded research, information/education campaigns, national renewable energy plans, regulations/standards, guaranteed markets, green pricing such as the 'standard offer' to buy solar electricity at a premium by the Ontario government [START_REF]Community + Small Project Support: Standard Offer-Expanding Renewable Energy Opportunities[END_REF], and financial incentives such as 100,000 roof program in Germany [START_REF] Weiss | The German PV solar power financing schemes reflected on the German PV market[END_REF]), expanded the manufacturing of PV, they are largely inconsequential in the energy budget of an expanding world. Both industry and government are moving slowly towards the solar energy transition, but PV electrical production still makes up much less than 1% of the total world energy market. A catalyst to speed the up scaling of PV manufacturing is needed [START_REF] Pearce | Using the Internet to Reduce Market Risk for Alternative Energy Sources: The Case of Large-Scale Solar Photovoltaic Production[END_REF][START_REF] Pearce | Catalyzing Mass Production of Solar Photovoltaic Cells Using University Driven Green Purchasing[END_REF]. Most global governments are financially secure enough to take advantage of this opportunity and become this necessary medium of change.

There has been significant progress in the last 30 years in the performance of solar PV. Solar cells made from a variety of materials have demonstrated efficiencies of over 10% and are currently manufactured globally. In the U.S., the PV industry has grown at average of ~20%/year, a growth rate comparable to that of the semiconductor and computer industries [START_REF]Solar electric power[END_REF]. This growth rate, while large, must be viewed in context of the global energy market; in 2004 the peak electrical generation capacity in the world was 3,736 GW [15] while the total global installed solar PV was only 1 GW.

The Photovoltaic Market

The market for solar PV continues to expand rapidly. World solar photovoltaic market installations rose to 1.46 GW in 2005, representing annual growth of 34% and leading to a cumulative installed solar PV electricity generating capacity of 5GW (up 39%) [START_REF] Solarbuzz | World PV Industry Report Highlights: World Solar PV Market[END_REF]. As the production volume of PV has increased, the price per module fell rapidly [START_REF] Shah | Photovoltaic technology: The case for thin-film solar cells[END_REF]. This trend continued for several years with prices in dollars per peak Watt ($/W p ) dropping to $4.94/W p in May 2004 in the U.S. [START_REF] Solarbuzz | Solar Module Price Environment[END_REF] and then beginning to climb to $5.47/W p as of December 2006 as the worldwide demand for PV outstripped the supply of purified silicon used in the majority of "1 st Generation" solar modules. The market will eventually correct this problem as more silicon feedstock factories scale up and more "2 nd Generation" factories are built that use small quantities of amorphous silicon in thin films or non-silicon based PV technologies. The rapid increases in PV even at values over 25% per year are not enough to make a serious run at fossil fuel supplies for decades. This limit to the growth is largely because the cost of solar electricity fully installed at even $3/Wp is $0.09/kW-hr is just competitive with the average retail price of electricity (in the U.S. in 2006 it was $0.1047/kW-hr [START_REF]Average Retail Price of Electricity to Ultimate Customers by End-Use Sector[END_REF]). It should be noted that to secure significant market share PV will need to compete with wholesale electricity rates, which are considerably lower than retail rates. The cost of the PV module represents 50-60% of the total installed cost of a PV system so the cost of solar electricity will only enjoy market dominance if the price of installed solar systems can be brought down to about $1/Wp after enormous scale up in manufacturing. At this price, the payback from savings for a consumer's electric bill is Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002 about five years in locations with utility costs of more than $0.12/ kW-hr. This will greatly increase the demand for solar PV panels on residential and commercial rooftops.

Scale and Photovoltaic Production

This ambitious target of $1/W p can be reached by one of two ways. The first method would demand a major scientific breakthrough -a radically new and efficient method of converting light into electricity. There continues to be world wide effort in new "3 rd Generation" [START_REF] Green | Third Generation Photovoltaics: Advanced Solar Conversions[END_REF] types of solar cells that improve upon existing cell designs and recruit new materials/nanotechnology that have potential to do this. Such unknown breakthroughs are not necessary if the second method is employed, which utilizes a massive scale-up of PV manufacturing facilities. A "Solar City Factory", a factory capable of making 2.1-3.6 GW of solar cells per year would achieve new economies of scale to reduce the price by the necessary ~3X factor [START_REF] Keshner | Study of Potential Cost Reductions Resulting from Super-Large-Scale Manufacturing of PV Modules[END_REF].

It is not enough just to build a 3GW factory to obtain the necessary cost reductions. The Solar City Factory would achieve such cost decreases by utilizing five key principles to drive down the cost of "2 nd Generation" solar cells [START_REF] Keshner | Study of Potential Cost Reductions Resulting from Super-Large-Scale Manufacturing of PV Modules[END_REF]. First, all materials including the substrate, packaging, and materials would need to be designed to last for > 25 years in realistic environmental conditions so that the capital cost of the modules can be sufficiently amortized. Second, the transportation and handling of substrate materials should be minimized and automated similar to the design of semiconductor chip facilities. This reduces the steps for fabrication based on cleaning, and thus also reduces breakage and costs to improve yields. Third, all of the material inputs entering the solar cell factory should be optimized for solar cell production. This can only occur if the demand for the materials is large enough to warrant a dedicated line. For example, substrate glass specifically manufactured without iron for solar cells can increase the sunlight entering the cell by about fifteen percent. At current solar PV manufacturing lines for thin films altering the glass recipe for small batches is uneconomic, but this would be reversed at scales of 100X current PV manufacturing lines. Fourth, multiple lines of identical equipment with rotating planned downtime for cleaning and maintenance could be used to minimize unplanned downtime and reduce the cost of maintenance and operations. Finally, the need for the most expensive constituent materials could be minimized by increasing the utilization rates for high purity input materials, which will both reduce costs directly and improve availability of rare materials for some types of solar cells (e.g. indium).

These improvements and the massive scale up necessary to make them, require an enormous capital investment in a single plant of ~ $600 million [START_REF] Keshner | Study of Potential Cost Reductions Resulting from Super-Large-Scale Manufacturing of PV Modules[END_REF]. To put this level of investment into perspective, it is helpful to note that investment in new plants to manufacture solar cells exceeded $1 billion in 2005 globally [START_REF] Solarbuzz | World PV Industry Report Highlights: World Solar PV Market[END_REF]. This money was primarily invested in 1 st Generation (poly-silicon and single crystal silicon) PV manufacturing plants, which, while they benefit from larger scales, do not have the economic advantages of 2 nd Generation PV technology and can not obtain the economic leverage outlined by the plan above. Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002

Although both the semiconductor and flat panel display factories cost well over $1 billion each, even large companies have been reluctant to invest so much into a single plant, because solar PV is often viewed as a somewhat risky investment. The government support that has helped the solar PV industry substantially and conventional energy prices have historically been unstable. A large investor able to reduce the risk is needed to invest the money necessary to build a multi-GW solar PV manufacturing facility.

Multi-GW Solar PV Production 3.1. Government Policies for multi-GW PV Production

Most of the world's governments are well positioned to take advantage of this opportunity to be the investor that produces the first multi-GW PV fabrication facility. Any government with the necessary political stability and available cash flow could look at the production facility as an investment in the same way that government reserves are kept in banks to earn interest. The investment would yield direct financial return after the facility was producing solar panels for public sale. The government would enjoy national benefits including a good reputation throughout the world, increased national energy security and long-term economic growth [START_REF] Estaintot | European commission-supported R&D activities in the field of photovoltaics[END_REF].

Increases in national energy security would come from both an increase in domestic supply of energy (and thus a reduction in present and future energy imports), but also an increase in national electric grid robustness. PV has been established to be part of the solution to deliver firm, dependable power during extreme peak conditions leading to outages [START_REF] Perez | PV and Grid Reliability: Availability of PV Power during Capacity Shortfalls[END_REF]. Solar PV electric production has the advantage over more conventional forms of electricity supply of being a distributed energy source -solar PV systems can be located near the consumer by placing solar panels on any surface that is exposed to sunlight without large shading losses during the day. As a distributed electrical generation source, this technology acts as a network (much like the Internet), and is therefore much less susceptible to large-scale power outages caused by natural or manmade disasters.

Long-term economic growth would come first from domestic reduction of electricity costs, which would drive down the cost of most products; and second by providing a technological head start in a job-intense industry with an enormous and growing international demand. There is considerable evidence that the integration of PV into the electric supply could reduce electricity costs for all consumers. This is a consequence of the fortunate solar irradiance-demand correlation, where most human activities tend to follow the sunlight cycle. For example, residential PV installations in New England have shown that ~60% of the total daily output from PV systems is supplied during the utility peak period when the utility system load is 95% of the maximum load ≥ reached during the day [START_REF] Bzura | The New England Electric Photovoltaic Systems Research and Demonstration Project[END_REF]. Solar PV thus provides value to the grid by displacing generation from conventional units that use higher-cost fuels. At low solar electricity percentages, or penetration levels, the least efficient and most costly sources of conventional electricity can be removed. This would drive down both economic and environmental costs for the entire electrical production system. Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002

Many utilities have actively tried to prevent any transition to a more distributed grid because it would represent a reduction in revenue from sales of their principle product. In addition, they have been particularly unenthusiastic for intermittent renewable energy sources such as solar PV. This is due to the fact that at certain penetration levels the savings offered by intermittent generations could be offset by the increase in costs of partial loads of reserves, making it uneconomic. Particularly in the case of solar energy, the high costs have prevented the large distribution of solar electricity on purely economic grounds.

In the case explored here, where large scale production of PV drives down the costs to be competitive with conventional sources, only technical criteria (e.g. intermittency) are valid for the determination of PV penetration levels. Although higher penetration levels of PV generation can be problematic, corrective measures such as assigning more generating units to regulating duty or installing fast-response combined-cycle generators have been available for decades [START_REF] Anderson | The Effect of Photovoltaic Power Generation on Utility Operation[END_REF]. The problem of fluctuating cloud conditions can be solved simply by deploying solar PV over a larger area; if a network of PV installations is dispersed over 100 km 2 the tolerable penetration increases to 18.1%, and if the area is 1,000 km 2 the limit is 35.8% [START_REF] Jewell | Limits on Cloud-induced Fluctuation in Photovoltaic Generation[END_REF]. In a review of several studies, an upper penetration limit of PV generation of 16% is estimated for a system load set by the worst-case cloud pattern [START_REF] Wan | Factors Relevant to Utility Integration of Intermittent Renewable Technologies[END_REF]. Combined, these factors have enormous potential economic return for any of the world's governments. The first plant at 2GW/year would have revenues of $2 billion/year and as additional plants replicate the first successful demonstration the positive effect on even the largest developed economies could be staggering. With construction of this single plant, the country would be the dominant PV manufacturer in the world, out-producing the aggregate of global manufacturers. The country would have a firm technological lead in large-scale PV manufacturing, which it can leverage to rapidly deploy several multi-GW plants to feed the enormous global market, which could have a powerful effect on most country's balance of trade. Even within the country, any energy source supplanted with PV would increase the number of people employed with solar, as investment in solar PV produces substantially higher levels of employment than equivalent investments in conventional energy supplies [START_REF]Comparative Analysis of Employment from Air Emission Reduction Measures[END_REF]. A study for Environment Canada found that for every $1 million invested, an average of 17 jobs are created in the PV energy sector compared to only 7.3 jobs created from the same investment in conventional energy [START_REF]Comparative Analysis of Employment from Air Emission Reduction Measures[END_REF].

The overall investment of $600 million is relatively modest compared to the budget of most of the world's governments or even yearly expenditures in a single class. For example, >70 countries spend more than $600 million/year on their military budgets each [29]. A developed government could also raise money for the multi-GW PV plant investment from the following sources: i) reallocation of the international aid for a single year, ii) increased taxes or decreased spending by a fraction of a percent, iii) a donation drive to get each of citizen to donate a modest amount. This donation could also be in the form of a "solar bond" that would pay dividends to citizen investors. In developing countries $600 million represents a larger relative investment, but it still within the reach of Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002 most countries. There is significant public support for environmental protection [START_REF] Inglehart | Public Support for Environmental Protection: Objective Problems and Subjective Values in 43 Societies[END_REF] and based on the public's recent historical environmental record and opinion data [START_REF] Iniyan | A survey of social acceptance in using renewable energy sources for the new millennium[END_REF], considerable public support for such a bold initiative would be expected in many countries.

Policies To Protect Domestic PV Manufacturers

It is understandable that any proposal to have a government directly interfere in a market to the point of becoming an actual competitor would be a major concernparticularly for solar cell producers. The concern would focus on the probability of disrupting the market and destroying small, medium, and even large businesses if they were slated to compete in solar cell production with the government, which has significantly larger resources. Based upon relatively well-founded estimates of the modules produced in a very large multi-GW facility, it is likely that the government's PV modules will be a 3X less expensive and, given the scale of the plant, provide ~ 50%-100% of the global market. This would necessarily drive many producers out of business if these cells were immediately placed on the market. In order to avoid this problem, the government could purchase all of their own PV cells for the time that is necessary to begin production from another multi-GW manufacturing plant by existing national PV companies. The government-manufactured cells could be integrated into the roofs of all government buildings and then, when roof space had been completely exhausted, PV could be integrated into south-facing highway sound barriers and into carports constructed over all government parking lots. Similarly, off-grid government electrical applications could be completely solar powered (e.g. highway signs, buoys, bus stops and street lights). Again, this would be an investment for the government. Purchasing solar panels at such a low cost per W p has a very competitive return because of the avoided electrical costs. For example, utilizing conservative assumptions such as 2.4 hours of peak sunlight on average per day, a 1kW system would output 876kW-hrs worth $70 at $0.08/kW-hr and have a simple payback of 14 years. Over the lifetime of the system, estimated at 25 years, this is a rate of return > 5%. This return on investment, while a conservative estimate because it assumes that the price of energy escalates at the rate of inflation, is competitive with mortgage rates.

During this transition period where the government could set a future date when the PV from the facility would be put on the open market (e.g. 2-4 years). By giving the PV industry enough lead time to ramp up their own production facilities, the government will not be directly competing against them for customers because the demand will far outstrip the supply of a single digit multi-GW plant at such low $/Wp prices [START_REF] Payne | Accelerating PV expansion: Supply analysis for competitive electricity markets[END_REF]. Domestic manufacturers would i) know that it is technically feasible to produce inexpensive solar cells in such a large plant, ii) receive government technical support with designs and construction of multi-GW factories based on the lessons learned in funding, building, and operating a plant, and iii) an enormous incentive to scale up immediately before being driven out of business by not only the government fabrication facility, but also manufacturing plants being built by their competitors.

Industrial Symbiosis in PV Production

Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002

In order to increase both the economic and environmental performance of the manufacturing sector, the government could introduce a second set of policies with the large scale PV manufacturing in order to facilitate the widespread adoption of industrial symbiosis. In industrial symbiosis, traditionally separate industries are considered collectively to gain competitive advantage by instituting the mutually-beneficial physical exchange of materials, energy, water, and/or by-products. Such a system collectively optimizes material and energy use at efficiencies beyond those achievable by any individual process alone. The key benefits of industrial symbiosis are collaboration and the synergies offered by geographic proximity [START_REF] Chertow | Industrial Symbiosis: Literature and Taxonomy[END_REF]. Industrial symbiotic systems such as the now-classic network of companies in Kalundborg, Denmark have spontaneously evolved from a series of micro-innovations over a long time scale. In order to accelerate the process and demonstrate the possibilities for mutually-beneficial collaboration, the government could engineer the design of the new solar PV manufacturing plant using industrial symbiosis.

The multi-GW PV factory could sit at the center of a next generation eco-industrial park. The eco-industrial park would be made up of at least 8 symbiotic factories as seen in Fig. 1. These could be located outside of a major population center to provide raw materials, labor, and a ready market. The first factory would be a conventional recycling facility [START_REF]The Intergovernmental Panel on Climate Change[END_REF]. In this way, the glass and aluminum needed to fabricate the solar cell could be recovered from recycled materials and thus have a lower embodied energy (95% lower for aluminum and 20% for glass) [START_REF] Milne | Embodied energy[END_REF].

The raw glass from the recycling plant will be fed to a sheet glass factory (2) and melted using natural gas. Generally, the high quality requirements of flat glass prohibit the use of post-consumer waste glass [START_REF] Ruth | An industrial ecology of the US glass industry[END_REF]. However, the glass industry is exploring a method to eliminate problems with color contamination by using thin plastic coatings, which can be made with a variety of colors and that would vaporize during re-melting without affecting the quality of the new glass [START_REF] Bisio | Encyclopedia of Energy Technology and the Environment[END_REF]. This would benefit the industry considerably because using recycled glass, called cullet, has several important benefits that include: i) lowering the consumption of raw materials, ii) reducing the release of CO 2 formed in the chemical reaction of raw materials, iii) increasing the life of the furnace by up to 30% due to lower melting temperatures, iv) reducing energy use during the melting stage of production and thus reducing additional GHGs and operating costs, and v) reducing the costs associated with pollution abatement due to lower emissions of NO x , SO 2 and particulates [START_REF] Ruth | An industrial ecology of the US glass industry[END_REF][START_REF] Papke | Glass Recycling and Reuse From Municipal Wastes[END_REF][START_REF]Glass Container Recycling Stays at Record High in 1995[END_REF]. The factory will output cut sheets of 3 mm thick glass with seamed edges and low iron content in order to obtain a high solar transparency. Finally, the glass will be tempered for mechanical strength and coated with a transparent conductor such as tin oxide, zinc oxide, or indium tin oxide to be used as the substrates.

The production stages in the glass factory that utilize large amounts of heat will have integrated thermal recovery to provide lower grade heat for the other facilities and a greenhouse complex (3a). In the greenhouse complex, exotic plants can be grown year Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002 round in even northern climates utilizing the waste heat from the manufacturing plants in the eco-industrial park. Similarly, waste heat could be utilized to provide grow rooms for mushrooms (3b). In both agricultural plants, the food or other agricultural products will be sold outside of the park and the growing medium will be provided by the recycling facility (1), compost for the greenhouse (3a) and wood pulp or compost for the mushroom growing facility (3b). In warmer climates, the waste heat could be used to drive absorption chillers perhaps providing cooling for an office park.

The substrates will then be fed directly into the PV module plant (4). Then a group of semiconductor and metal thin-film deposition systems will create and pattern the active layers of the solar cells. All waste semiconductors and metals will be captured and returned to a semiconductor recycling plant [START_REF] Hansen | Greenhouse Gas Growth Rates[END_REF] to supplement the incoming and generally expensive high purity materials going into the deposition systems. The output of the PV deposition and patterning lines will be PV solar panels ready for protective coatings and packaging.

The aluminum extracted from common drinking cans in the recycling center (1) will be fed to an aluminum fabrication factory (6) that will produce coated aluminum rails for holding the glass solar panels. The aluminum rails will be extruded and used to provide a simple and inexpensive means of attachment to rooftops, ground mounted systems, or building integrated PV. In addition, the extruded aluminum rails could be designed into ground and flat roof mounting balance of system components. Similar to the glass manufacturing plant [START_REF]The Intergovernmental Panel on Climate Change. Summary for Policymakers, Climate Change 2001: Impacts, Adaptation, and Vulnerability[END_REF], waste heat will be recovered and used in the symbiotic collective or to heat the greenhouse (3a) or mushroom grow rooms (3b).

Next, in the packaging factory [START_REF] Pearce | Photovoltaics -A Path to Sustainable Futures[END_REF], the solar panels are interconnected if necessary and are sprayed with a protective polymer coating to seal them to the environment. The primary constituents of the polymer coating could again be acquired from the recycling plant (1) and common plastics. The panels would then be wired with quick connects so they can be easily installed in the field by connecting to each other, an inverter, or battery bank.

Finally, the panels would be packaged for shipment to prevent damage in cardboard boxes and cushioned with shredded newspaper. The newspapers would again come from the recycling plant (1) and the cardboard could come from a cardboard plant [START_REF] Pearce | Net Energy Analysis For Sustainable Energy Production From Silicon Based Solar Cells[END_REF], which would gain its raw materials from the recycling factory [START_REF]The Intergovernmental Panel on Climate Change[END_REF].

By co-locating these factories in the eco-industrial park, the transportation costs and energy between them can be minimized and many of the inputs for the solar PV plant can literally come from waste products in the surrounding population centers. It should be noted that each factory will be scaled appropriately for the symbiotic system and should be individually profitable so that independent businesses can replicate this model by colocating and benefit from industrial symbiosis in future facilities. Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002

The Catalytic Effect

The primary focus of this technological/policy assemblage is to provide a reproducible demonstration of large-scale mass manufacturing of PV. This demonstration and the accompanying policies will provide a fertile ground for companies already experienced in PV manufacturing and those businesses considering the PV industry (e.g. GE) to begin large-scale mass production themselves. Both the technical demonstration and the secure business environment will help reduce the financial risk that prospective multi-GW manufacturers face when considering such large investments. Following the policy suggestions, numerous multi-GW PV facilities would be built. This in term would drive further private sector investment in the complimentary industries such as power conditioning (inverters and charge controllers) and storage (batteries). In order to target the primary global issue of climate change, the country would either need to be the supplier for the electrical production equipment for the entire world or, more likely, spread the concept to other nations. This expansion could take place following existing models such as country to country collaborative work to solve environmental problems such as the Canada-China Cooperative Project in Clean Production [START_REF] Schwartz | Canada's role in Chinese environmental protection[END_REF].

Limitations to Approach and Potential Solutions

Photovoltaic electricity production is not the complete solution to global energy requirements, nor will it single-handedly deliver us from the potential enormous harm of destabilizing the global climate. PV can, however, tip the balance into a renewable distributed energy future where many green technologies and aggressive conservation can lead society towards a sustainable state. PV cannot do it alone because there are inherent limitations to PV technology itself. The most important of which is intermittency; PV does not produce useful quantities of electricity at night and under heavy cloud conditions. There are two approaches that can be used to mitigate this limitation.

First, a form of storage can be used in which electricity is produced in excess during the day and stored for use at night in devices such as batteries. This drives up both first costs and recurring costs from maintenance/battery replacement (~ 5 years for current technologies). Solar cells themselves operate maintenance-free and are normally warranted for 25 years or more. To avoid the need for chemical batteries, it is also possible to couple solar cells with other conventional sources. For example, in a solar PV-hydro hybrid system, the solar modules produce power during the day with the hydro plant holding the water in reserve until the night, when it takes over power production [START_REF] Schwartz | Canada's role in Chinese environmental protection[END_REF]. In addition, at night or during cloudy weather, electrical demand can be met by retaining the existing fossil fuel power stations, which are already built and largely paid for, and starting them up as required. The cost of using existing power stations in this way is surprisingly low since the vast majority of the cost of running a power station is the fuel cost.

Second, with low PV penetration levels solar arrays can eliminate the need for batteries by utilizing an inverter to use the grid as a storage medium. This method is limited Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002 to about 20% before the grid will have to be altered to adapt to intermittent sources. Studies have shown that penetration limits usually are not technical limits but economic limits where additional operational costs are greater than the added value of intermittent renewable energy technologies -so the 20% rule of thumb should be viewed as a moving target [START_REF] Wan | Factors Relevant to Utility Integration of Intermittent Renewable Technologies[END_REF]. An added economic advantage for utilities using solar for symbiotic energy production with other sources, rests on fortunate daily and yearly solar timing. When electrical demand is highest for many electrical suppliers (during the summer for air conditioning) PV power output is at or near its peak. Thus, solar power facilities can be used to mitigate peak demand and thus hold down costs for current energy suppliers. Unfortunately, for this same reason, as the solar penetration level is increased, some of the economic benefits disappear as an increasing number of conventional generating units are partially loaded to add load-following capability and operating reserve to the system. This can be detrimental to overall grid efficiency because of the increased cycling duties and decreased efficiency caused by the partial loading of these generators, which will increase fuel consumption and maintenance costs.

In the long term, to move to a more renewable and thus intermittent energy mix, the grid must be transformed. Electricity-producing units that can quickly power up and down to meet the demand not handled by intermittent sources like PV and wind need to be developed. Technologies such as gas-fired microturbines, fuel cells, other combined heat and power (CHP) devices, and plug-in hybrid vehicles all combined or working independently offer possibilities to overcome the technical challenge of penetration limits.

Conclusions

This paper has shown that the first multi-GW thin film solar PV facility at the heart of a government sponsored eco-industrial park could be an enormous benefit to both the economy and environment. The main objective of the policies outlined here, however, was not to benefit from the output of a single multi-GW PV plant. Rather, the primary focus of these procedures is to provide a reproducible demonstration of large-scale mass manufacturing of PV to encourage domestic PV manufacturers to scale up production and thus drive down production costs globally. Both the technical demonstration and the secure business environment provided by the technical and policy assembly outlined here will help reduce the financial risk that prospective multi-GW manufacturers face when considering such large investments and help catalyze a true renewable energy transition.

Figure 1 .

 1 Figure Captions Figure 1. Graphical Representation of the Eco-Industrial Park to produce multiple GigaWatts of Solar PV

Joshua M. Pearce, "Industrial Symbiosis for Very Large Scale Photovoltaic Manufacturing", Renewable Energy 33, pp. 11011108, 2008. http://dx.doi.org/10.1016/j.renene.2007.07.002