- Email: [email protected]
Photovoltaic systems for current and future applications
Solar Energy. Vol. 41. NO. 5. pp. 465---1.73. 1988 Printed in the US.A.
0038-092X/88 $3.00 + .00 Copyright e3 1988 Pergamon Press plc
PHOTOVOLTAIC SYSTEMS FOR CURRENT AND FUTURE APPLICATIONS* H. N. POST and M. G. THOMAS Sandia National Laboratories, Division 6223, P.O. Box 5800. Albuquerque, NM 87185, U.S.A. Abstract--This article examines photovoltaic power system applications, including remote standalone, dispersed grid-connected, and large generation centers. Photovoltaic system options for both current and future applications are described and costs for each of these options are developed. The results of this examination show that future applications will utilize the system technology available today and subsystem technology advances can be accommodated through minor system changes.
1. I N T R O D U C T I O N
Terrestrial photovoltaic (PV) system technology has matured dramatically during the past decade. The partnership of U.S. industry and the U.S. Department of Energy National Photovoltaics Program has developed an extensive database on system technology. This database is capable of providing information on the design, installation, subsystem integration, operation, and energy value appropriate for systems in each of the major power application sect o r s - r e m o t e , dispersed grid-connected, and large generation centers. Today, thousands of PV systems exist worldwide providing power to small remote grid-independent applications such as telecommunications and vaccine refrigeration where photovoltaics is the economical power option of choice. Although a few prototype multimegawatt PV power plants have been installed, collector costs still preclude widespread deployment in the large generation sector. Even so, the system technology necessary to incorporate lower-cost collector technologies does exist and is reasonably fixed[ 1]. Here, an important distinction should be made between system technology and subsystem component technology. Subsystem components--collectors, power conditioners, tracking drives, batteries, etc.--are continually improving (i.e., higher efficiencies, lower costs, and improved reliabilities). The system-level technology necessary to meld these components into a viable source of power for each of the application sectors has reached maturity and is unlikely to change even though the components may. This article discusses the photovoltaic system options for both current and future applications. System configurations for current applications are based on state-of-the-art information on hardware, performance, and cost derived from the National PV Program as well as private projects. Most of these configurations represent actual systems that are installed in the field. On the other hand, system configurations
*This work supported by the U.S. Department of Energy, Division of Photovoltaic Energy Systems Technology.
proposed for future applications were developed through array field optimization studies conducted through the DOE PV Program. Collector specifics such as efficiency, operating characteristics, and costs projected for future applications are based on the authors' expectations of collector technology appropriate to the mid-1990s.
2. T O D A Y ' S M A R K E T P L A C E
Many PV power systems are economically viable for today's applications. The cost of PV power without on-site storage is currently in the range of 30 to 40 cents/kWh[2]. This compares favorably with all other generators for remote power and is approaching competitiveness with consumer costs of peak grid power at some locations in the United States. In addition, PV power systems have proven to be reliable with projected operation and maintenance costs verified by field experience[3,4]. Most of the PV systems sold today are designed to provide power to remote, grid-independent, or standalone applications such as telecommunications, vaccine refrigeration, security lighting, cathodic protection, battery charging, and water pumping. These systems are typically configured as shown in Fig. 1, and include straightforward designs that are sized to meet whatever availability the user needs[5]. Recent analyses have shown how economic viability can be demonstrated for these systems. Essentially, system requirements are used to define the PV system design as well as the design of a competitive energy source and the value to the user is estimated. The results of this type of analysis are shown in Fig. 2 for five remote standalone applications. This figure displays the relatively large competitive range enjoyed by PV systems under current economic conditions. Another major application sector is the grid-connected system. The economic viability of these systems is also tied to the cost of the competition, which in this case is the utility. Because costs of grid power are usually much lower than that for remote power, the constraints on competitiveness are more stringent. With the consumer cost of grid energy averaging 8 cents/kWh in the United States, we find that
465
H. N. POSTand M. G. THOMAS
466
ARRAY SUBSYSTEM
'
~
~
LOAD SUBSYSTEM
POWER BATTERY
~
~
CHARGE
~
CO
"
~"
)l-
L
BATTERY MODULES
BATTERY SUBSYSTEM
Fig. 1. Block diagram of a standalone photovoltaic system. the cost of PV-generated energy is currently a factor of 4 to 5, too high for baseload use. However, with continued progress toward collector cost reduction, we anticipate that photovoltaics will be competitive with some conventional sources of grid energy during the 1990s[6]. Grid-connected systems, as depicted in Fig. 3, can be of two general types. The first is a system asso-
ciated with an on-site load such as a residence. This type of system does not require on-site storage but does require dc to ac inversion and utility compatibility. With this type of system, energy is sold to the utility during times of excess generation and energy is bought from the utility during times of shortfall, usually at night. The second type of grid-connected system is the large generation center that simply sells
WATER PUMPING (25 meters head) 0
25 m,~/day
0
S kWh/day
:):U m~/aay
COMMUNICATIONS Z,z I(wn/aey
QJA
LW~
~
Ami~u
Q
VACCINE REFRIGERATION 0
SMALL
24 fit~
LARGE
,~ liter
LAHr~'H
LIGHTING & HOME POWER U
~)MALL
I If.l~nl
MrCUlUM
.: fl,~nt.~
LAMf~lt
~ Eight.t, 04 r~rQit#
MULTI-USE SYSTEMS U
2 kWl'~lOay
• PI" A r ~ i r J~ ~ I I e T
ID Kwn,'Oay
#~r larx~, n,fnXe1~l~" J~e In p e c ' u h a n , ~ m thv 14"~M, I d Ht.u#k O r x u n f : m ~ n ~p~rlr~ ,rHpn~
Lir4ptl.3f:p~
Photovoltaic system applications
467
./ /
: :.:'.....~l
I [l[tll~blPJ
f j7
Fig. 3. Schematic of a grid-connected system.
energy to the utility or is utility owned. This system is ground mounted and is typically several hundred kilowatts to multimegawatts in size. The economic viability limits for grid-connected systems is clouded by several factors. These include the type of financing used to build the system, the negotiated purchase prices for energy to and from the utility, and, for utility ownership, the marginal value given to the PV generated energy. Although these issues are not well defined today, tens of utilities have purchased PV systems for evaluation, and a considerable number of homeowners have equipped their residences with grid-connected PV systems. With this in mind, it would appear that current costs are certainly approaching the viability threshold for grid-tied systems. Many technological advances are occurring for PV subsystem components that will lead to lower costs and the expansion of economic viability limits for all the application sectors. The efficiencies of crystalline silicon PV cells have increased dramatically to over 22% for one-sun cells and greater than 25% for cells under concentration. New thin-film devices indicate that the early-on stability problems associated with amorphous silicon collectors may be substantially reduced through the use of multijunction devices, thinner films, and other materials. Concepts for concentrating PV systems continue to show promise. Power conditioning designs show improvements in effi-
ciency and reduced costs through advances in power electronics technology. And, most importantly, none of these improvements should affect the system technology already in place. They will simply lower the costs of the systems. 3. REMOTE STANDALONE APPLICATIONS AS noted earlier, most of the PV systems currently installed fall in this application sector. These systems are typically configured to include array, battery, and load subsystems as shown in Fig. 1, although the battery subsystem may be deleted for certain applications. The system technology necessary to design, install, integrate, operate, and value the energy for these systems is well understood. Simplified design techniques have been developed that allow the designer to size the system based on the availability desired by the user. In addition, a major study to evaluate the cost and performance of international standalone PV projects was recently completed by Meridian Corporation under contract to Sandia National Laboratories. Experiences with over 2700 systems in 45 countries covering five application categoriesmwater pumping, communications, vaccine refrigeration, lighting and home power, and multiuse systemsmwere evaluated[3]. The key finding from this study is that these systems have been well accepted by the users based on their reliability, independence from fuel, and
468
H. N. POSTand M. G. THOMAS
minimal maintenance requirements. The PV arrays were found to be reliable and although the performance of power conditioning and end-use equipment was somewhat less than the arrays, careful selection of field-proven components should ensure successful system operation. Similar results were obtained in a detailed survey conducted by the New Mexico Solar Energy Institute of over 100 PV water pumping systems installed in New Mexico. The Photovoltaic Design Assistance Center at Sandia has completed a major effort to develop a design practices manual oriented toward standardized point designs for each of the viable standalone applications. This manual covers not only the standardized designs but also the simplified design methodologies, installation plans, maintenance considerations, and economic methodologies associated with these designs[14]. By far, the largest standalone application area is communications. However, water pumping and vaccine refrigeration offer significant new markets for photovoltaics, especially in lesser-developed countries. During August 1986, three PV-powered water pumping systems were installed in Bolivia by Solavolt International through a cooperative effort between the World Bank, the U.S. Department of Energy, and the government of Bolivia (see Fig. 4). These systems were installed to demonstrate the viability and advantages of PV water pumping to both the government of Bolivia, who has a great need for more systems, and to the World Bank, a major international lending institution. A similar effort to install six vaccine refrigeration systems at health clinics in Central America was completed in April 1987. This project, cosponsored by the Organization of
American States. the U.S. Department of Energy, and the Pan American Health Organization demonstrates the many potential benefits of PV refrigeration in an area of the world that has great need for many more systems. A summary of the costs of these two system types is presented in Table 1. Future standalone applications are not anticipated to change dramatically from today's options. As collector costs for PV systems decrease and operating costs of competitive energy options increase, the economic viability limits of PV systems for all standalone applications are expected to expand greatly. As this market increases, the system technology to be utilized in meeting future applications should remain the same as in place today.
4. DISPERSED GRID-CONNECTED APPLICATIONS More information exists about dispersed systems than any other application sector. During the last eight years, the DOE National Photovoltaic Program through Sandia National Laboratories has been involved with the detailed evaluation of dozens of systems ranging in size from approximately 1 kW to systems greater than 200 kW. Many of the smaller systems, 1 to 6 kW, have been evaluated at regional experiment stations located at Cape Canaveral, Florida and Las Cruces, New Mexico. Nine larger systems, 18 to 225 kW, located throughout the country have also been included in the evaluation. All of the smaller systems have been roof mounted, using fixed fiat-plate modules and sized to meet typical residential loads. The roof mounting takes advantage of the existing structure to minimize costs,
Fig. 4. Photovoltaic water pumping system installed at Sanka Jahuira, Bolivia.
Photovoltaic system applications
469
Table 1. Cost summary for PV standalone applications Water pumping PV modules (160 W) Hardware (submersible pump/dc motor. structure, cabling, etc.) Installation Water tanks (6 m3--3 days storage) Shipping Total
Vaccine refrigeration $ 960
PV modules (160 W) Hardware (4 ft3 refrigerator, structure, 2200 battery housing, etc.) 270 Installation 2000 Batteries(3 kWh--6 days storage) 70 Shipping $5500 Total
to avoid land constraints, and to minimize conflict with local zoning ordinances. Because the residential sector represents a significant amount of the electricity used in the United States and will continue to do so, this application represents a potentially large market. Whether homeowners will choose to provide part of their energy is an issue beyond the scope of this article. Nonetheless, detailed design and feasibility methods exist. Historically, this type of system was assumed to be too expensive for widespread adoption. These systems were so small that design and installation costs became a significant factor. In addition, for the customer who bought one system, economies of scale were not available. Recent work, however, has eliminated these penalties. A detailed design study conducted by MIT Energy Laboratory under contract to Sandia has provided standardization designs for residential systems. Thirty homes were retrofitted with PV systems using this design within the New England Electric service area over the past year (see Fig. 5)[7]. These systems were installed for about S0.60 Wac, which is somewhat less than comparable large systems. The same design was utilized for all 30 systems so that design and installation costs were spread over all the systems. Finally, by purchasing the modules for all of the systems in one procurement, much of the benefit of economy of scale was realized. This
$ 960 970 200 300 70 $2500
same type of design can be used with any pitched roof home and is compatible with the newly developing thin film modules. Other types of applications exist for dispersed systems. These include schools, shopping centers, hospitals, and business complexes in the ten to several hundred kilowatt size range and like the residential systems are, for the most part, roof mounted. Whether they are roof or ground mounted, the design practices for these systems are well understood and have no unique design requirements. An interesting application of dispersed systems is demonstrated in a recent installation in Phoenix, Arizona (see Fig. 6). Here, a builder, John Long Homes, has installed nearly 200 kW of fiat-plate modules to provide power for a housing subdivision. This facility requires no special technology but is an innovative way to utilize standard design practice and take advantage of economies of scale. Neither this application nor any of the others described have any difficulty incorporating today's subsystem technology or any other anticipated for the future. 5. LARGEGENERATIONCENTER APPLICATIONS As with the other application sectors, the system technology for large generation centers is well known. However, the current costs of PV systems preclude their use in the bulk power market. Although several
Fig. 5. Residential photovoltaic system at Gardner, MA.
470
H.N. Post and M. G. THOMAS
Fig. 6. John Long Homes photovoltaic system.
utilities have installed PV power plants, these installations are primarily for evaluation purposes and to allow utility personnel to gain hands-on experience with photovoltaic technology. As costs come down, especially in the area of collector technology, photovoltaics will be competitive with conventional bulk power generation. The question that is ad-
dressed here is which PV system option is most cost effective for the future large (> 1 MW) generation center application. Five competing system options are examined, based on current technology as well as future technology expected in the mid-1990s. These five options include fixed, one-axis tracking and two-axis tracking
Photovoltaic system applications flat-plate collectors as well as concentrator collectors utilizing linear and point focus Fresnel optics. These options are compared for a high insolation site (Albuquerque) where PV would most likely be utilized for large-size multimegawatt power plants. Energy performance for the five collector options was computed using the PVFORM model developed at Sandia[8]. This model represents state of the art in PV performance simulation and has been verified for accuracy by comparison with measured data at several sites around the country. The system options are compared using three different figures-of-merit. The first is defined by the total installed system cost (S/m") divided by the annual energy production (kWhaJm'-) from the system. The annual energy production is determined by the product of available insolation to the collector (kWh/m") and the annual ac conversion efficiency ('qAAc) of the system. The second figure-of-merit is the levelized energy cost ($/kWhac) in constant 19865. This figure-of-merit is outlined in the DOE Five Year Research Plan and the equations utilized here are identical to those presented in that plan. Finally, the last figure-of-merit is the commonly used initial installed cost per peak watt. Current system configurations are based on existing field installations, prototype hardware, and studies reflecting current capabilities. The fixed flat-plate system uses a modular design developed for the DOE PV Program by Hughes Aircraft Company[9]. The one-axis tracking flat-plate option is based on the SMUD PV1 and PV2 systems installed adjacent to the Rancho Seco Nuclear Plant in California. These systems totaling 2 MW use north-south oriented horizontal PV panels supported by a pedestal-torque tube
471
structure. The two-axis tracking flat-plate option is based on the ARCO Solar Lugo power plant installed near Hesperia, CA. This field, shown in Fig. 7, is approximately 1 MW in size and uses heliostat-type trackers. The concentrator options reflect designs developed through the DOE program. The system using a point focus Fresnel optics module (84x concentration) was developed by Intersol Power Corporation. The system using a line focus Fresnel optics module (40x concentration) was developed by Entech Corporation. The future system options are based on array field designs typically 100 MW in size developed through the DOE PV program. The fixed flat-plate configuration is based on a design developed by BattelleColumbus Laboratories[10]. This design uses large panels supported by precast concrete pedestals. Both the one-axis and two-axis tracking flat-plate configurations are based on designs developed by Hughes Aircraft Company[l 1]. The one-axis option, shown in Fig. 8, uses horizontal north-south panels controlled by freon-actuated drive units. The two-axis configuration uses a low-profile, pedestal-supported array driven by an electromechanical tracker. The point-focus Fresnel concentrator option uses an improved module (500× concentration) mounted to a low-profile, pedestal-supported tracking structure. Similarly, the line-focus Fresnel concentrator option uses an improved, 20× concentration module. Cost estimates for the five system options for both the current and mid-1990s technologies are presented in Table 2. The annual ac efficiency is based on a 0.89 field dc to ac conversion factor and a 0.85 peak ac to average ac conversion factor[12]. Crystalline silicon modules estimated to cost $4.90/Wp.,: are used
Fig. 7. ARCO Solar Lugo array field.
472
H . N . POST and M. G. THO~,IAS
Fig. 8. Prototype one-axis tracking array installed in Sandia Test Facility. Albuquerque. New Mexico. with the current-flat-plate options. The power conditioning cost is estimated at $0.20/Wp.~, for current applications and $0.10/'Wp.~c for future applications. The latter estimate is based on utilizing an advanced 5 - M W power conditioner design by GE. The concentrator module costs were estimated at $ 3 . 9 0 / W p . ~ and $3.40/Wp.~c for the point focus and line focus Fresnel collectors, respectively. In all cases, a fixed indirect cost of 25% has been assumed based on cur-
rent system procurement experience. Mid- 1990s cost effectiveness is based on the realization of a 14% peak dc efficient advanced flat-plate module costing $ 1 . 0 0 / Wp.ac, Similarly, peak dc efficiencies of 22% and 18% are proposed for the high concentration and low concentration module options, respectively. The results of Table 2 show that the current levelized energy cost for the line focus Fresnel concentrator offers a small advantage over the other four
Table 2. Cost comparison--Albuquerque site Current technology Available Array Installed cost Levelized Cost per insolation System Module PCS BOS Indirect Total per annual energy energy cost peak watt (kWh/m 2) "q~,.~,c (S/m:) ( S / m : ) ( S / m : ) ( S / m : ) (S/m:) (S/kWh.,) (S/kWh~,) ($/Wp.~,) Fixed flat plate l-Axis flat plate 2-Axis flat plate Concentrator (PFF/84 x ) Concentrator (LFF/40 x )
2458 2955 3365
0.08 0.08 0.08
476 476 476
19 19 19
85 135 200
145 158 174
725 788 869
3.69 3.33 3.23
0.32 0.29 0.28
7.49 8.14 8.97
2610
0.11
487
26
200
178
891
3.10
0.28
6.75
2610
0.10
392
25
200 154 771 Mid-1990s technology
2.95
0.26
6.25
Fixed flat plate 1-Axis flat plate 2-Axis flat plate Concentrator (PFF/500 × ) Concentrator (LFF/20×)
2458 2955 3365
0.10 0.10 0.10
125 125 125
12 12 12
50 77 124
47 54 65
234 268 326
0.95 0.91 0.97
0.09 0.08 0.09
1.90 2.18 2.65
2610
0.16
145
19
140
76
380
0.91
0.08
1.96
2610
0.13
125
16
124
66
331
0.98
0.09
2.09
Photovoltaic system applications options at a favorable southwestern U.S. site. These data, however, have not been verified in an actual installation. This trend also holds true for the other figures-of-merit. However, for future applications, all five options have levelized energy costs which are nearly identical at 8 to 9 cents/kWh~¢. Several previous studies indicate that even at 20 to 25 cents/ kWh~¢, photovoltaics will become an economical option for power production in every application sector including bulk power[ 13]. The level ized energy costs computed in Table 2 would indicate that current systems are rapidly approaching cost viability and future systems offer a competitive alternative for bulk power production. 6. CONCLUSIONS Current and future photovoltaic power system applications have been described in this article. For today, standalone applications for telecommunications, vaccine refrigeration, water pumping, etc. are economical and reliable choices. Virtually any modestly sized ( < 1 kW) remote application today can be met competitively with PV technology. System designs, operational strategies, and system value are known and are appropriate to any future applications in this sector. Photovoltaic system energy costs are between $0.30 and 0 . 4 0 / k W h without storage. Although these costs are competitive with remote power, they have yet to reach parity with the grid. Nonetheless, tens of smaller dispersed systems associated with homes and businesses and several larger generation centers have been installed. With only small decreases in costs, the gridtied systems will become much more of a reality. In any case, the system technology of today is adequate to accommodate any anticipated subsystem improvements for the future grid-connected application,
2. 3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
REFERENCES*
1. W. J. Stolte, The Integration of 1990s Thin-Film Technologies Into Photovoltaic System Design. SAND86-
*Sandia and DOE reports are available from the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161.
13.
14.
473
7052 (Albuquerque: Sandia National Laboratories, February, 1987). Work performed by Bechtel National, Inc. National Photovoltaics Program: Five-Year Research Plan 1987-1991, DOE/CH 10093-7, U.S. Department of Energy. Washington, D.C., May t 1987). D. Eskenazi, D. Kerner, and L. Slominski, Evaluation of International Photovoltaic Projects, SAND85-7018 (Albuquerque: Sandia National Laboratories, September 1986). Work performed by Meridian Corporation. Reliabili~' of Intermediate-Sized Photovoltaic Systems. SAND86-7039 (Albuquerque: Sandia National Laboratories, January 1987). Work performed by New Mexico Solar Energy Institute. R. N. Chapman, Design considerations for stand-alone photovoltaic systems. Proc. Symposium on Applications of Solar and Renewable Energy-86, Cairo, Egypt, p. 130, March (1986). H. N. Post, D. E. Arvizu, and M. G. Thomas. A comparison of photovoltaie system options for today's and tomorrow's technologies. Proc. 18th IEEE PV Specialists Conf.. Las Vegas, NV, p. 1353, October (1985). M. C. Russell and E. C. Kern. Stand-OffBuilding Block Systems for Roof-Mounted Photovoltaic Arrays. SAND85-7020 (Albuquerque: Sandia National Laboratories, June 1986). Work performed by MIT Energy Laboratory. D. F. Menicucci, "PVFORM--A new approach to photovoltaic system performance modelling. Proc. 18th IEEE PV Specialists Conference. Las Vegas, NV, p. 614, October 1985. Modular Photovoltaic Array Field. SAND83-7028 IAIbuquerque: Sandia National Laboratories, September 1984). Work performed by Hughes Aircraft Company. G. T. Noel et al., Optimization and Modularity" Study for Large-Size Photovoltaic Flat-Panel Arrav Fields. SAND84-7012 (Albuquerque: Sandia National Laboratories. September 1986). Work performed by Battelle-Columbus Laboratories. J. A. Castle. Modular Array Field Designs for Tracking Flat Plate Photovoltaic Systems. SAND86-7036 (Albuquerque: Sandia National Laboratories, June 1987). Work performed by Hughes Aircraft Company. M. G. Thomas, The Value of PV System Experiments: Volume 1. A Preliminary Assessment of the Lessons Learned from Nine Intermediate-Sized Systems. SAND84-0900/1 (Albuquerque: Sandia National Laboratories. August 1984). G. Bonk, G. Jones, and M. Thomas. The Effect of Energy Scenarios on Photovoltaic System Worth. SAND817012 (Albuquerque: Sandia National Laboratories, June 1982). Work performed by General Electric Company. Stand-Alone Photovoltaic Systems--A Handbook of Recommended Design Practices, SAND 87-7023 (Albuquerque: Sandia National Laboratories, April 1988).
0038-092X/88 $3.00 + .00 Copyright e3 1988 Pergamon Press plc
PHOTOVOLTAIC SYSTEMS FOR CURRENT AND FUTURE APPLICATIONS* H. N. POST and M. G. THOMAS Sandia National Laboratories, Division 6223, P.O. Box 5800. Albuquerque, NM 87185, U.S.A. Abstract--This article examines photovoltaic power system applications, including remote standalone, dispersed grid-connected, and large generation centers. Photovoltaic system options for both current and future applications are described and costs for each of these options are developed. The results of this examination show that future applications will utilize the system technology available today and subsystem technology advances can be accommodated through minor system changes.
1. I N T R O D U C T I O N
Terrestrial photovoltaic (PV) system technology has matured dramatically during the past decade. The partnership of U.S. industry and the U.S. Department of Energy National Photovoltaics Program has developed an extensive database on system technology. This database is capable of providing information on the design, installation, subsystem integration, operation, and energy value appropriate for systems in each of the major power application sect o r s - r e m o t e , dispersed grid-connected, and large generation centers. Today, thousands of PV systems exist worldwide providing power to small remote grid-independent applications such as telecommunications and vaccine refrigeration where photovoltaics is the economical power option of choice. Although a few prototype multimegawatt PV power plants have been installed, collector costs still preclude widespread deployment in the large generation sector. Even so, the system technology necessary to incorporate lower-cost collector technologies does exist and is reasonably fixed[ 1]. Here, an important distinction should be made between system technology and subsystem component technology. Subsystem components--collectors, power conditioners, tracking drives, batteries, etc.--are continually improving (i.e., higher efficiencies, lower costs, and improved reliabilities). The system-level technology necessary to meld these components into a viable source of power for each of the application sectors has reached maturity and is unlikely to change even though the components may. This article discusses the photovoltaic system options for both current and future applications. System configurations for current applications are based on state-of-the-art information on hardware, performance, and cost derived from the National PV Program as well as private projects. Most of these configurations represent actual systems that are installed in the field. On the other hand, system configurations
*This work supported by the U.S. Department of Energy, Division of Photovoltaic Energy Systems Technology.
proposed for future applications were developed through array field optimization studies conducted through the DOE PV Program. Collector specifics such as efficiency, operating characteristics, and costs projected for future applications are based on the authors' expectations of collector technology appropriate to the mid-1990s.
2. T O D A Y ' S M A R K E T P L A C E
Many PV power systems are economically viable for today's applications. The cost of PV power without on-site storage is currently in the range of 30 to 40 cents/kWh[2]. This compares favorably with all other generators for remote power and is approaching competitiveness with consumer costs of peak grid power at some locations in the United States. In addition, PV power systems have proven to be reliable with projected operation and maintenance costs verified by field experience[3,4]. Most of the PV systems sold today are designed to provide power to remote, grid-independent, or standalone applications such as telecommunications, vaccine refrigeration, security lighting, cathodic protection, battery charging, and water pumping. These systems are typically configured as shown in Fig. 1, and include straightforward designs that are sized to meet whatever availability the user needs[5]. Recent analyses have shown how economic viability can be demonstrated for these systems. Essentially, system requirements are used to define the PV system design as well as the design of a competitive energy source and the value to the user is estimated. The results of this type of analysis are shown in Fig. 2 for five remote standalone applications. This figure displays the relatively large competitive range enjoyed by PV systems under current economic conditions. Another major application sector is the grid-connected system. The economic viability of these systems is also tied to the cost of the competition, which in this case is the utility. Because costs of grid power are usually much lower than that for remote power, the constraints on competitiveness are more stringent. With the consumer cost of grid energy averaging 8 cents/kWh in the United States, we find that
465
H. N. POSTand M. G. THOMAS
466
ARRAY SUBSYSTEM
'
~
~
LOAD SUBSYSTEM
POWER BATTERY
~
~
CHARGE
~
CO
"
~"
)l-
L
BATTERY MODULES
BATTERY SUBSYSTEM
Fig. 1. Block diagram of a standalone photovoltaic system. the cost of PV-generated energy is currently a factor of 4 to 5, too high for baseload use. However, with continued progress toward collector cost reduction, we anticipate that photovoltaics will be competitive with some conventional sources of grid energy during the 1990s[6]. Grid-connected systems, as depicted in Fig. 3, can be of two general types. The first is a system asso-
ciated with an on-site load such as a residence. This type of system does not require on-site storage but does require dc to ac inversion and utility compatibility. With this type of system, energy is sold to the utility during times of excess generation and energy is bought from the utility during times of shortfall, usually at night. The second type of grid-connected system is the large generation center that simply sells
WATER PUMPING (25 meters head) 0
25 m,~/day
0
S kWh/day
:):U m~/aay
COMMUNICATIONS Z,z I(wn/aey
QJA
LW~
~
Ami~u
Q
VACCINE REFRIGERATION 0
SMALL
24 fit~
LARGE
,~ liter
LAHr~'H
LIGHTING & HOME POWER U
~)MALL
I If.l~nl
MrCUlUM
.: fl,~nt.~
LAMf~lt
~ Eight.t, 04 r~rQit#
MULTI-USE SYSTEMS U
2 kWl'~lOay
• PI" A r ~ i r J~ ~ I I e T
ID Kwn,'Oay
#~r larx~, n,fnXe1~l~" J~e In p e c ' u h a n , ~ m thv 14"~M, I d Ht.u#k O r x u n f : m ~ n ~p~rlr~ ,rHpn~
Lir4ptl.3f:p~
Photovoltaic system applications
467
./ /
: :.:'.....~l
I [l[tll~blPJ
f j7
Fig. 3. Schematic of a grid-connected system.
energy to the utility or is utility owned. This system is ground mounted and is typically several hundred kilowatts to multimegawatts in size. The economic viability limits for grid-connected systems is clouded by several factors. These include the type of financing used to build the system, the negotiated purchase prices for energy to and from the utility, and, for utility ownership, the marginal value given to the PV generated energy. Although these issues are not well defined today, tens of utilities have purchased PV systems for evaluation, and a considerable number of homeowners have equipped their residences with grid-connected PV systems. With this in mind, it would appear that current costs are certainly approaching the viability threshold for grid-tied systems. Many technological advances are occurring for PV subsystem components that will lead to lower costs and the expansion of economic viability limits for all the application sectors. The efficiencies of crystalline silicon PV cells have increased dramatically to over 22% for one-sun cells and greater than 25% for cells under concentration. New thin-film devices indicate that the early-on stability problems associated with amorphous silicon collectors may be substantially reduced through the use of multijunction devices, thinner films, and other materials. Concepts for concentrating PV systems continue to show promise. Power conditioning designs show improvements in effi-
ciency and reduced costs through advances in power electronics technology. And, most importantly, none of these improvements should affect the system technology already in place. They will simply lower the costs of the systems. 3. REMOTE STANDALONE APPLICATIONS AS noted earlier, most of the PV systems currently installed fall in this application sector. These systems are typically configured to include array, battery, and load subsystems as shown in Fig. 1, although the battery subsystem may be deleted for certain applications. The system technology necessary to design, install, integrate, operate, and value the energy for these systems is well understood. Simplified design techniques have been developed that allow the designer to size the system based on the availability desired by the user. In addition, a major study to evaluate the cost and performance of international standalone PV projects was recently completed by Meridian Corporation under contract to Sandia National Laboratories. Experiences with over 2700 systems in 45 countries covering five application categoriesmwater pumping, communications, vaccine refrigeration, lighting and home power, and multiuse systemsmwere evaluated[3]. The key finding from this study is that these systems have been well accepted by the users based on their reliability, independence from fuel, and
468
H. N. POSTand M. G. THOMAS
minimal maintenance requirements. The PV arrays were found to be reliable and although the performance of power conditioning and end-use equipment was somewhat less than the arrays, careful selection of field-proven components should ensure successful system operation. Similar results were obtained in a detailed survey conducted by the New Mexico Solar Energy Institute of over 100 PV water pumping systems installed in New Mexico. The Photovoltaic Design Assistance Center at Sandia has completed a major effort to develop a design practices manual oriented toward standardized point designs for each of the viable standalone applications. This manual covers not only the standardized designs but also the simplified design methodologies, installation plans, maintenance considerations, and economic methodologies associated with these designs[14]. By far, the largest standalone application area is communications. However, water pumping and vaccine refrigeration offer significant new markets for photovoltaics, especially in lesser-developed countries. During August 1986, three PV-powered water pumping systems were installed in Bolivia by Solavolt International through a cooperative effort between the World Bank, the U.S. Department of Energy, and the government of Bolivia (see Fig. 4). These systems were installed to demonstrate the viability and advantages of PV water pumping to both the government of Bolivia, who has a great need for more systems, and to the World Bank, a major international lending institution. A similar effort to install six vaccine refrigeration systems at health clinics in Central America was completed in April 1987. This project, cosponsored by the Organization of
American States. the U.S. Department of Energy, and the Pan American Health Organization demonstrates the many potential benefits of PV refrigeration in an area of the world that has great need for many more systems. A summary of the costs of these two system types is presented in Table 1. Future standalone applications are not anticipated to change dramatically from today's options. As collector costs for PV systems decrease and operating costs of competitive energy options increase, the economic viability limits of PV systems for all standalone applications are expected to expand greatly. As this market increases, the system technology to be utilized in meeting future applications should remain the same as in place today.
4. DISPERSED GRID-CONNECTED APPLICATIONS More information exists about dispersed systems than any other application sector. During the last eight years, the DOE National Photovoltaic Program through Sandia National Laboratories has been involved with the detailed evaluation of dozens of systems ranging in size from approximately 1 kW to systems greater than 200 kW. Many of the smaller systems, 1 to 6 kW, have been evaluated at regional experiment stations located at Cape Canaveral, Florida and Las Cruces, New Mexico. Nine larger systems, 18 to 225 kW, located throughout the country have also been included in the evaluation. All of the smaller systems have been roof mounted, using fixed fiat-plate modules and sized to meet typical residential loads. The roof mounting takes advantage of the existing structure to minimize costs,
Fig. 4. Photovoltaic water pumping system installed at Sanka Jahuira, Bolivia.
Photovoltaic system applications
469
Table 1. Cost summary for PV standalone applications Water pumping PV modules (160 W) Hardware (submersible pump/dc motor. structure, cabling, etc.) Installation Water tanks (6 m3--3 days storage) Shipping Total
Vaccine refrigeration $ 960
PV modules (160 W) Hardware (4 ft3 refrigerator, structure, 2200 battery housing, etc.) 270 Installation 2000 Batteries(3 kWh--6 days storage) 70 Shipping $5500 Total
to avoid land constraints, and to minimize conflict with local zoning ordinances. Because the residential sector represents a significant amount of the electricity used in the United States and will continue to do so, this application represents a potentially large market. Whether homeowners will choose to provide part of their energy is an issue beyond the scope of this article. Nonetheless, detailed design and feasibility methods exist. Historically, this type of system was assumed to be too expensive for widespread adoption. These systems were so small that design and installation costs became a significant factor. In addition, for the customer who bought one system, economies of scale were not available. Recent work, however, has eliminated these penalties. A detailed design study conducted by MIT Energy Laboratory under contract to Sandia has provided standardization designs for residential systems. Thirty homes were retrofitted with PV systems using this design within the New England Electric service area over the past year (see Fig. 5)[7]. These systems were installed for about S0.60 Wac, which is somewhat less than comparable large systems. The same design was utilized for all 30 systems so that design and installation costs were spread over all the systems. Finally, by purchasing the modules for all of the systems in one procurement, much of the benefit of economy of scale was realized. This
$ 960 970 200 300 70 $2500
same type of design can be used with any pitched roof home and is compatible with the newly developing thin film modules. Other types of applications exist for dispersed systems. These include schools, shopping centers, hospitals, and business complexes in the ten to several hundred kilowatt size range and like the residential systems are, for the most part, roof mounted. Whether they are roof or ground mounted, the design practices for these systems are well understood and have no unique design requirements. An interesting application of dispersed systems is demonstrated in a recent installation in Phoenix, Arizona (see Fig. 6). Here, a builder, John Long Homes, has installed nearly 200 kW of fiat-plate modules to provide power for a housing subdivision. This facility requires no special technology but is an innovative way to utilize standard design practice and take advantage of economies of scale. Neither this application nor any of the others described have any difficulty incorporating today's subsystem technology or any other anticipated for the future. 5. LARGEGENERATIONCENTER APPLICATIONS As with the other application sectors, the system technology for large generation centers is well known. However, the current costs of PV systems preclude their use in the bulk power market. Although several
Fig. 5. Residential photovoltaic system at Gardner, MA.
470
H.N. Post and M. G. THOMAS
Fig. 6. John Long Homes photovoltaic system.
utilities have installed PV power plants, these installations are primarily for evaluation purposes and to allow utility personnel to gain hands-on experience with photovoltaic technology. As costs come down, especially in the area of collector technology, photovoltaics will be competitive with conventional bulk power generation. The question that is ad-
dressed here is which PV system option is most cost effective for the future large (> 1 MW) generation center application. Five competing system options are examined, based on current technology as well as future technology expected in the mid-1990s. These five options include fixed, one-axis tracking and two-axis tracking
Photovoltaic system applications flat-plate collectors as well as concentrator collectors utilizing linear and point focus Fresnel optics. These options are compared for a high insolation site (Albuquerque) where PV would most likely be utilized for large-size multimegawatt power plants. Energy performance for the five collector options was computed using the PVFORM model developed at Sandia[8]. This model represents state of the art in PV performance simulation and has been verified for accuracy by comparison with measured data at several sites around the country. The system options are compared using three different figures-of-merit. The first is defined by the total installed system cost (S/m") divided by the annual energy production (kWhaJm'-) from the system. The annual energy production is determined by the product of available insolation to the collector (kWh/m") and the annual ac conversion efficiency ('qAAc) of the system. The second figure-of-merit is the levelized energy cost ($/kWhac) in constant 19865. This figure-of-merit is outlined in the DOE Five Year Research Plan and the equations utilized here are identical to those presented in that plan. Finally, the last figure-of-merit is the commonly used initial installed cost per peak watt. Current system configurations are based on existing field installations, prototype hardware, and studies reflecting current capabilities. The fixed flat-plate system uses a modular design developed for the DOE PV Program by Hughes Aircraft Company[9]. The one-axis tracking flat-plate option is based on the SMUD PV1 and PV2 systems installed adjacent to the Rancho Seco Nuclear Plant in California. These systems totaling 2 MW use north-south oriented horizontal PV panels supported by a pedestal-torque tube
471
structure. The two-axis tracking flat-plate option is based on the ARCO Solar Lugo power plant installed near Hesperia, CA. This field, shown in Fig. 7, is approximately 1 MW in size and uses heliostat-type trackers. The concentrator options reflect designs developed through the DOE program. The system using a point focus Fresnel optics module (84x concentration) was developed by Intersol Power Corporation. The system using a line focus Fresnel optics module (40x concentration) was developed by Entech Corporation. The future system options are based on array field designs typically 100 MW in size developed through the DOE PV program. The fixed flat-plate configuration is based on a design developed by BattelleColumbus Laboratories[10]. This design uses large panels supported by precast concrete pedestals. Both the one-axis and two-axis tracking flat-plate configurations are based on designs developed by Hughes Aircraft Company[l 1]. The one-axis option, shown in Fig. 8, uses horizontal north-south panels controlled by freon-actuated drive units. The two-axis configuration uses a low-profile, pedestal-supported array driven by an electromechanical tracker. The point-focus Fresnel concentrator option uses an improved module (500× concentration) mounted to a low-profile, pedestal-supported tracking structure. Similarly, the line-focus Fresnel concentrator option uses an improved, 20× concentration module. Cost estimates for the five system options for both the current and mid-1990s technologies are presented in Table 2. The annual ac efficiency is based on a 0.89 field dc to ac conversion factor and a 0.85 peak ac to average ac conversion factor[12]. Crystalline silicon modules estimated to cost $4.90/Wp.,: are used
Fig. 7. ARCO Solar Lugo array field.
472
H . N . POST and M. G. THO~,IAS
Fig. 8. Prototype one-axis tracking array installed in Sandia Test Facility. Albuquerque. New Mexico. with the current-flat-plate options. The power conditioning cost is estimated at $0.20/Wp.~, for current applications and $0.10/'Wp.~c for future applications. The latter estimate is based on utilizing an advanced 5 - M W power conditioner design by GE. The concentrator module costs were estimated at $ 3 . 9 0 / W p . ~ and $3.40/Wp.~c for the point focus and line focus Fresnel collectors, respectively. In all cases, a fixed indirect cost of 25% has been assumed based on cur-
rent system procurement experience. Mid- 1990s cost effectiveness is based on the realization of a 14% peak dc efficient advanced flat-plate module costing $ 1 . 0 0 / Wp.ac, Similarly, peak dc efficiencies of 22% and 18% are proposed for the high concentration and low concentration module options, respectively. The results of Table 2 show that the current levelized energy cost for the line focus Fresnel concentrator offers a small advantage over the other four
Table 2. Cost comparison--Albuquerque site Current technology Available Array Installed cost Levelized Cost per insolation System Module PCS BOS Indirect Total per annual energy energy cost peak watt (kWh/m 2) "q~,.~,c (S/m:) ( S / m : ) ( S / m : ) ( S / m : ) (S/m:) (S/kWh.,) (S/kWh~,) ($/Wp.~,) Fixed flat plate l-Axis flat plate 2-Axis flat plate Concentrator (PFF/84 x ) Concentrator (LFF/40 x )
2458 2955 3365
0.08 0.08 0.08
476 476 476
19 19 19
85 135 200
145 158 174
725 788 869
3.69 3.33 3.23
0.32 0.29 0.28
7.49 8.14 8.97
2610
0.11
487
26
200
178
891
3.10
0.28
6.75
2610
0.10
392
25
200 154 771 Mid-1990s technology
2.95
0.26
6.25
Fixed flat plate 1-Axis flat plate 2-Axis flat plate Concentrator (PFF/500 × ) Concentrator (LFF/20×)
2458 2955 3365
0.10 0.10 0.10
125 125 125
12 12 12
50 77 124
47 54 65
234 268 326
0.95 0.91 0.97
0.09 0.08 0.09
1.90 2.18 2.65
2610
0.16
145
19
140
76
380
0.91
0.08
1.96
2610
0.13
125
16
124
66
331
0.98
0.09
2.09
Photovoltaic system applications options at a favorable southwestern U.S. site. These data, however, have not been verified in an actual installation. This trend also holds true for the other figures-of-merit. However, for future applications, all five options have levelized energy costs which are nearly identical at 8 to 9 cents/kWh~¢. Several previous studies indicate that even at 20 to 25 cents/ kWh~¢, photovoltaics will become an economical option for power production in every application sector including bulk power[ 13]. The level ized energy costs computed in Table 2 would indicate that current systems are rapidly approaching cost viability and future systems offer a competitive alternative for bulk power production. 6. CONCLUSIONS Current and future photovoltaic power system applications have been described in this article. For today, standalone applications for telecommunications, vaccine refrigeration, water pumping, etc. are economical and reliable choices. Virtually any modestly sized ( < 1 kW) remote application today can be met competitively with PV technology. System designs, operational strategies, and system value are known and are appropriate to any future applications in this sector. Photovoltaic system energy costs are between $0.30 and 0 . 4 0 / k W h without storage. Although these costs are competitive with remote power, they have yet to reach parity with the grid. Nonetheless, tens of smaller dispersed systems associated with homes and businesses and several larger generation centers have been installed. With only small decreases in costs, the gridtied systems will become much more of a reality. In any case, the system technology of today is adequate to accommodate any anticipated subsystem improvements for the future grid-connected application,
2. 3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
REFERENCES*
1. W. J. Stolte, The Integration of 1990s Thin-Film Technologies Into Photovoltaic System Design. SAND86-
*Sandia and DOE reports are available from the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161.
13.
14.
473
7052 (Albuquerque: Sandia National Laboratories, February, 1987). Work performed by Bechtel National, Inc. National Photovoltaics Program: Five-Year Research Plan 1987-1991, DOE/CH 10093-7, U.S. Department of Energy. Washington, D.C., May t 1987). D. Eskenazi, D. Kerner, and L. Slominski, Evaluation of International Photovoltaic Projects, SAND85-7018 (Albuquerque: Sandia National Laboratories, September 1986). Work performed by Meridian Corporation. Reliabili~' of Intermediate-Sized Photovoltaic Systems. SAND86-7039 (Albuquerque: Sandia National Laboratories, January 1987). Work performed by New Mexico Solar Energy Institute. R. N. Chapman, Design considerations for stand-alone photovoltaic systems. Proc. Symposium on Applications of Solar and Renewable Energy-86, Cairo, Egypt, p. 130, March (1986). H. N. Post, D. E. Arvizu, and M. G. Thomas. A comparison of photovoltaie system options for today's and tomorrow's technologies. Proc. 18th IEEE PV Specialists Conf.. Las Vegas, NV, p. 1353, October (1985). M. C. Russell and E. C. Kern. Stand-OffBuilding Block Systems for Roof-Mounted Photovoltaic Arrays. SAND85-7020 (Albuquerque: Sandia National Laboratories, June 1986). Work performed by MIT Energy Laboratory. D. F. Menicucci, "PVFORM--A new approach to photovoltaic system performance modelling. Proc. 18th IEEE PV Specialists Conference. Las Vegas, NV, p. 614, October 1985. Modular Photovoltaic Array Field. SAND83-7028 IAIbuquerque: Sandia National Laboratories, September 1984). Work performed by Hughes Aircraft Company. G. T. Noel et al., Optimization and Modularity" Study for Large-Size Photovoltaic Flat-Panel Arrav Fields. SAND84-7012 (Albuquerque: Sandia National Laboratories. September 1986). Work performed by Battelle-Columbus Laboratories. J. A. Castle. Modular Array Field Designs for Tracking Flat Plate Photovoltaic Systems. SAND86-7036 (Albuquerque: Sandia National Laboratories, June 1987). Work performed by Hughes Aircraft Company. M. G. Thomas, The Value of PV System Experiments: Volume 1. A Preliminary Assessment of the Lessons Learned from Nine Intermediate-Sized Systems. SAND84-0900/1 (Albuquerque: Sandia National Laboratories. August 1984). G. Bonk, G. Jones, and M. Thomas. The Effect of Energy Scenarios on Photovoltaic System Worth. SAND817012 (Albuquerque: Sandia National Laboratories, June 1982). Work performed by General Electric Company. Stand-Alone Photovoltaic Systems--A Handbook of Recommended Design Practices, SAND 87-7023 (Albuquerque: Sandia National Laboratories, April 1988).