As the cost of photovoltaics (PV) drops, aesthetics will become the primary barrier to their acceptance. There is a new generation of PV modules that have been integrated with building materials to serve the dual function of keeping out the weather and providing electricity. Building integration eliminates the unsightly appearance of aluminum framed PV modules mounted on top of a roof. It also eliminates the added expense of racks and the labor to install them.

 This paper highlights 15 projects, 12 of which were completed within the last year, and features roof integrated PVs. These projects demonstrate how triple junction thin-film amorphous-silicon PV laminates bonded to metal roofing can lower overall cost and make installations aesthetically pleasing at the same time. The projects are primarily located on the Northern California Coast, which, in spite of less than ideal weather conditions, has been a testing ground for PVs since the early 70s.
 

1. INTRODUCTION

 The building integrated photovoltaic (BIPV) installations analyzed in this paper vary in size from 0.5 kW to 6.5 kW and are all residential projects. When the incentives available from the California Energy Commission (CEC) ($3/Watt) and the Utility Photovoltaic Group (UPVG) ($1.50/Watt) are included, the cost varied from $2.80 to $15 per Watt for complete systems. Typical PV installations using crystalline modules cost from $10 to $17 per Watt. The triple junction thin-film amorphous technology also has unprecedented levels of efficiency in diffuse light, as well as hot sun where it outperforms crystalline modules in real life conditions by as much as 30% - based on the purchased Watt rating.
 

2. BUILDING INTEGRATION

 Mounting PV panels on buildings has been done since they were first commercially available, but until recently, PV panels were made of glass and aluminum. Mounting these modules meant drilling holes in a south-facing roof and installing them on expensive racks. In the event of roof repair or reproofing, the PV installation, which in most cases outlasts the roofing, had to be removed. The highly reflective crystalline modules covering part of a roof are considered unsightly and prohibited in many areas. Building integration means that the solar features become part of the weatherproof skin of a building. They can mimic windows or skylights or they can become part of the actual roofing material. New j PV roofing panels to be turned into aesthetically pleasing electric generators that are non-reflective and require no roof penetrations. In addition, the overall cost is reduced because thin-film amorphous panels are less expensive, the roofing is part of the panel, and no mounting racks are required.
 

3. CURRENT PV TECHNOLOGY

 There are currently three different kinds of photovoltaic panels on the market: crystalline, polycrystalline and amorphous. Crystalline modules are produced by creating a molten ingot that is then sliced into individual cells. Polycrystalline modules are extruded in thin wafers, which are then cut into individual cells. Both crystalline and polycrystalline cells are very fragile and need to be mounted in an aluminum frame between a stiff substrate and a layer of glass. Thin-film amorphous photovoltaic are produced by vapor deposition on a variety of substrates. The panels used in the following installations use stainless steel foil as the substrate which makes the resulting laminate lightweight, flexible and unbreakable.
 

4. AMORPHOUS TECHNOLOGY

 The efficiency of thin-film amorphous-silicon has been its major drawback. But with the introduction of triple junction lamination where three different semi-transparent layers are vapor deposited to take advantage of a wider spectrum of light the efficiency has been increased to rival crystalline cells. In side-by-side real life tests triple junction thin-film amorphous has outperformed crystalline modules by as much as 30% per purchased Watt. This unprecedented efficiency is due in large part to the fact that, unlike crystalline panels, the voltage produced by thin-film does not decrease in hot sunny conditions. Amorphous technology also is more efficient in indirect or diffuse light. They don’t require expensive trackers and work well in fixed south-facing installations. The panels still produce energy when partially shaded because there is a bypass diode between each cell, unlike crystalline panels that stop producing even when one cell in a module is shaded.

 It takes up to 5 years for crystalline modules to produce the energy it took to make them. In contrast, it takes thin film amorphous panels from 2 months to one year to produce the energy used to make them. The new triple junction technology now carries a 20-year warranty and degrades less than crystalline technology.
 

5. EASE OF INSTALLATION

 The laminates are encapsulated in elastomer polymer. The resulting weatherproof laminate can be bonded to standing seam metal roofing and installed like any standing seam metal roofing. The wires can be terminated under the ridge cap or in the soffit, which eliminates the need for roof penetrations. The 120-Watt laminates are 16 inches (406 mm) wide and 18 feet (5.5 m) long. The large size of the panels reduces the number of electrical connections. The fact that the laminates are bonded to actual roofing panels eliminates the use of mounting brackets needed for conventional crystalline panel installations. It also eliminates the labor necessary to mount conventional panel racks. The laminates come in a neutral blue gray that blends well with a variety of roofing colors.
 

6. INCENTIVE PROGRAMS AND NETMETERING

 There are various incentive programs in different states. The two that are available in California are the Utility Photovoltaic Group (UPVG) and the California Energy Commission (CEC) buydown programs. The UPVG had a national buydown program that ended in September1999. This program paid manufacturers $1.90 per Watt for PV systems tied to major utilities. Some manufacturers passed this rebate on to the customers; others used the rebate in house. The CEC has a $54 million Emerging Renewables Buydown Program that started in January of 1998 and is divided into five 5 block grants. The first block grant provides $3 per Watt or 50% of the total system, whichever is less, for grid-connected renewable energy systems. The rebate decreases $.50 per Watt in each successive grant period. These two programs worked in conjunction in 1999 to significantly reduce the cost of PV roofing systems.

 The commercial net metering law was passed in January of 1999 in California. Net metering allows for a customer’s (residential and commercial) meter to run in both directions and the net kWh usage to be measured once a year. This means that customers are reimbursed for the electricity they produce at the same rate that the utility charges. The net metering law also provides a property tax exemption equal to the total retail cost of the installation. In addition, there is a federal investment tax credit (26 USC Sec. 48) of up to 10% for commercial installations and five-year accelerated depreciation for all solar energy equipment (26 USC Sec. 168). Net metering allows a PV system to use the utility grid as a battery, thus eliminating battery expense and maintenance, and reducing the time necessary to pay back the cost of the PV system.
 

7. TYPES OF SYSTEMS

7.1 Line Tie Systems

 Line tie systems use an inverter that synchronizes with the
utility grid to make a direct connection with the photovoltaic roof. Many states have net metering laws that require utilities to purchase power produced from renewable sources, such as PV systems. Line-tie systems take advantage of these laws by using the utility to avoid the expense and maintenance of a battery pack. When the PV system produces more energy than is used, the meter runs backwards. A system can be sized to totally eliminate utility bills. The downside of Line-tie systems is that they do not provide back-up power if the utility goes down.

7.2 Utility Interactive Systems

 A utility interactive system uses the utility as a backup to provide uninterrupted power. This means that a multifunctional inverter uses the grid to keep a battery pack charged during extended rainy and overcast periods. The inverter can also be programmed to automatically turn on a generator when there is an extended utility outage and the battery pack is depleted. True sinewave inverters then can synchronize with the utility to run the meter backwards when the PV system is producing more energy than is being used.

7.3 Independent High Voltage AC Systems

 An off grid PV system is capable of providing the same level of comfort and convenience as is available from the utility grid. Independent systems require batteries to store solar electricity for use at night or on overcast days. An inverter is necessary to change the low voltage direct current produced by the PV array to high voltage alternating current used for conventional electrical appliances. Some systems also incorporate the use of other renewable energy sources, such as wind or microhydro to supplement the output of the PV array and provide power when the sun is not shining. Some systems also include
a gas or diesel backup generator.

7.4 Independent Low Voltage DC Systems

 A PV roof produces low voltage direct current, which can be used to charge batteries and operate low voltage (12 V, 24V) DC lights and appliances directly. This system is ideally suited for vacation homes or small cabins and can incorporate the same backup as independent high voltage systems.
 

8. PV ROOFING PROJECTS

 All, but one, of the 15 residential systems use thin-film amorphous BIPV. The first three projects were done from two to five years ago and are included to provide a comparison of technologies. They were also installed before the CEC and UPVG incentive programs were available. The other 12 projects were all completed within the last year using triple junction thin-film amorphous-silicon laminates.

 Fig. 1: 3 kW barn reroofing

8.1 Project 1

 The project is a single-family home and is located in Albion, CA. The PV system consists of 28 120-Watt crystalline modules covering 270 sqft. (25 m²) of south facing roof area. Racks were used to hold the framed glass modules. The 3.3 kW utility interactive system uses a true sinewave 220V 8 kW inverter and a 40 kWh industrial battery pack. The installed cost of the system was $15.15 per Watt with no buydowns.

 This project is by far the most expensive, partially because of the extra cost of racks and the difficulty of working on an existing roof. The PV modules will outlast the composite shingle roof, which means the system will have to be dismantled for reproofing, which will be a significant expense. The roof also faces southeast and there are tall trees to the west that cause partial shading of the array on summer afternoons. When a crystalline module is partially shaded its output stops.

8.2 Project No. 2

 The utility interactive project is a barn roof that powers a single-family residence and loft apartment all year long and charges electric vehicles in the summer. It is located in Albion, CA. The system consists of 56 45-Watt single junction thin-film amorphous laminated glass modules, which replaced 700 sqft. of leaking barn roof. The 2.5 kW array was installed with volunteer labor on three consecutive weekends. The balance of system includes a 110 V 4 kW true sinewave inverter and a 24 kWh industrial battery pack. The installed cost was $6 per Watt with no buydown.

 The glass panels replaced roofing and required no racks but did need a lot of care in installation. There are still some leaks in the roof and some of the glass panels have cracks. These cracks seem to be related to expansion on hot summer days when the attic space was superheated and then the fog rolled in causing a 50º F (10ºC) degree or more temperature differential between the inside and the outside of the glass. The cracks all occurred on the inside layer and have not decreased the panels’ electrical output. One of the panels was hit by a rock and had to be replaced, but other than that, the system is still working fine after five years.

8.3 Project No. 3

 This installation is a new roof on a single-family residence located in Mendocino, CA. This house was new construction designed to face due south and accommodate 17 120-Watt triple junction thin film amorphous laminates.  The laminates are 16 inches (406 mm) wide and 18 feet (5.5 m) long and are bonded to architectural standing seam metal roofing pans. The utility interactive system was the first private residential installation of triple junction technology. The balance of system included a 110 V 4kW true sinewave inverter and a 16 kWh battery pack. The cost of the installed system was $8 per Watt. No buydowns were available.

 Fig. 2: 2 kW PV roof on new construction

8.4 Project No. 4

 The project is an independent low voltage DC system on the south-facing portion of a new cabin roof located in Laytonville, CA. The roof consists of eight 60-Watt laminates bonded to architectural standing seam metal roofing pans for a total of 500 Watts. The system includes an 8.5 kWh battery pack, which powers 12 Volt lights and appliances in the cabin. Because another part of the site is grid connected, the system qualified for the CEC buydown program but did not qualify for the UPVG rebate. The cost of the system, including batteries, before the rebate was $12 per Watt. The system cost was $9.72 per Watt including the rebate. The roofing panels with the PV laminates went up just as quickly as the non-PV roof consequently there was no additional cost for the installation of the PV panels. The only PV installation cost was for the wiring.

8.5 Project No. 5

 Fig. 3: Reroofing with PV panels

 The project included a reroofing of a single-family home in Elk, CA. The PV system consists of six 60-Watt laminates and six 120-Watt laminates. The resulting one kW array was divided into two portions. The 12 Volt 60-Watt modules were combined to operate a 12 V whole house fan that transfers heat to a rock storage bed. The 24 Volt 120-Watt modules were combined to charge a 5.3 kWh battery pack that incorporates a 1,500 Watt modified sine wave inverter to provide 110 AC for a pump, lights and appliances. The system qualified for the CEC Buydown, but not for the UVPVG rebate. The cost of the system before buydown was $13 per Watt, after the buydown it was $10.21 per Watt.

 The design of the system was challenging because many skylights and roof penetrations divided up the area available for PV panels. Using the PVs to operate a fan for heating purposes is an ideal match because the solar heat is available at the same time as the solar electricity.

8.6 Project No. 6

 This utility interactive project uses 105 17-Watt triple junction thin-film amorphous shingles on a barn roof to provide 1.8 kW to an adjoining single-family home located in Crescent City, CA. The balance of system includes a 5.5 kW true sine wave inverter and a 45 kWh battery pack. The inverter has a charger, which allows the utility to be used as a backup to charge the batteries in overcast weather. This project was not served by a qualified utility therefore no rebates were available. The project cost was $13.50 per Watt.

 Shingle installations present challenges because of multiple connections and roof penetrations necessary to combine large numbers of 17-Watt modules. Shingle installations require attic access to make these connections, which is fine in a barn installation with no insulation but presents problems in a house where roof insulation is needed. The PV shingles have the advantage of blending with the surrounding shingle roof and fitting the more horizontal space available.

8.7 Project No. 7

 This installation was a reroofing of an older home in Berkeley, CA. The project consists of eight 60-Watt laminates and 12 120-Watt laminates that were installed to conform to the irregular roof area available. The utility interactive system has a 4 kW true sinewave inverter and a small 8.5 kWh battery pack capable of providing power to the house for short utility outages. The system qualified for the CEC and UPVG buydowns. The cost of the system before rebate was $13 per Watt and $9.16 per Watt after the buydowns.

 Fig. 4: 2.4 kW PV reproofing

 The ridge of the roof runs north and south, which means the east part of the roof gets morning sun, and the west-facing roof gets afternoon sun. This lowers the performance of this system. The irregular shape of the roof made the installation of metal roofing difficult. But the biggest drawback is the utility’s inconsistent voltage, which dropped to 83 Volts when the system was being inspected. When the utility power fluctuates outside the specs required for the inverter, the inverter automatically disconnects from the grid and starts running off the batteries rather than putting energy into the grid in effect not allowing net metering to occur.

8. 8 Project No.8

 An attached garage was added to a single-family residence in Fort Bragg, CA to provide space for 20 120-Watt laminates bonded to structural standing seam metal roofing. The resulting 2.4 kW array operates a net metered grid interactive system that includes a 48 kWh battery pack and a 5.5 kW true sinewave inverter. The system qualified for both the CEC and the UPVG rebate. The cost before the rebates was $14 per Watt; it was $10.36 per Watt after the rebates.

 Fig. 5: 2.5 kW roof on garage addition

 The structural standing seam has an integrated batten and because all panels were the same length, the entire 2.4 kW south-facing array was installed in three hours. This is in contrast to reproofing jobs that were not specifically designed for the size of the PV laminates that can take up to 10 times as long to install. This project also has a backup diesel generator, which is not included in the cost of the PV system.

8.9 Project No. 9

 This new detached garage with second floor apartment located in Little River, CA has a 3 kW PV roof consisting of 24 128-Watt laminates. The utility interactive system has a 48 kWh battery pack and a 5.5 kW true sinewave inverter. The system qualified for both the CEC and the UPVG buydown. The cost was $11 per Watt before buydown and $6.80 per Watt after the rebate.

 Fig. 6: Detached garage with 3 kW BIPV roof.

 Because this was new construction, the south-facing roof was designed to accommodate the standard length of the laminates and the battery and inverter are housed in a space specifically designed to maintain optimal conditions.
 

8.10 Project No. 10

 This project is a solar charging station located in Albion, CA. The 3.6 kW array consists of 30 120-Watt laminates bonded to standard length structural standing seam metal roofing with integrated battens. All components for the line-tie, net metered system were purchased wholesale and the system was owner installed. It qualified for both CEC and UPVG buydowns. The cost before rebates was $6.90 per Watt and after $2.80 per Watt.

 Fig. 7: 3.6 kW charging array

 This is by far the least expensive project and reflects the wholesale cost of components, the potential for owner installation to reduce labor cost, and the economies of designing a roof to fit the exact dimensions of standard length modules. Three people installed the entire 3.6 kW array in less than three hours. Each 18 foot long (5.5 m) pan required only 12 screws in perlins that were spaced 3 feet (1 m) apart. No plywood deck is necessary with structural standing seam metal roofing.

8.11 Project No. 11

 This installation is on the new roof of a single-family residence and an adjacent shop building located in Mendocino, CA. The 4 kW PV array consists of 36 60-Watt laminates on the shop building and 32 60-Watt laminates on the south-facing roof of the residence. The utility interactive system includes a 48 kWh battery pack and a 110 V 5.5 kW true sinewave inverter. The system qualified for both the CEC and the UPVG buydown and cost $11 per Watt before the buydowns and $6.70 per Watt after the rebate.

 Fig. 8: 2 kW roof on new shop building with 2 kW
 on adjoining house

 The installation was challenging because the standard length panels were too short to cover the entire length of the roof so the panels had to be lapped over the lower portion of the roof. The reason for ordering standard length panels was to be able to ship them by common carrier and unload them by hand, which made the shipping a lot less expensive.

8.12 Project No. 12

 This independent high voltage system on a new residence in Hailey, ID, consists of 40 120-Watt PV laminates bonded to structural standing seam metal roofing. The balance of system includes a 110 Volt 5.5 kW true sinewave inverter and a 48 kWh battery pack. No buydowns were available for this system because it is off grid. The system cost was $9 per Watt.

 Fig. 9: 4.8 kW roof on passive solar house

 Line extensions used to be comparatively inexpensive but restructuring has separated energy production and energy transmission. Now line extensions can cost as much as $100,000 per mile ($60,000 per kilometer). This installation was located seven miles (11.2 km) from the closest utility pole. Consequently, going solar is a bargain.

8.13 Project No. 13

 This 6 kW installation is on the south-facing roof of a new strawbale residence located in Modesto, CA. The PV array consists of 50 120-Watt laminates bonded to architectural standing seam metal roofing pans. The line-tie net metered installation uses a 110 Volt 5.5 kW true sinewave inverter and does not include a battery pack. The system qualified for both the CEC and the UPVG buydown. The cost before buydowns was $8.59 per Watt and after the buydown $4.53 per Watt.

 Fig. 10: 6 kW PV roof on new strawbale house

 This residence is over 6,000 sqft. (557.5 m²) for two people. Normally two people would only need two kilowatts but the size of the home dictated the larger array to cover the increased electrical load.

8.14 Project No. 14

 This cohousing project in Oakland, CA consists of four buildings with seven different residences. Three of the buildings are new construction designed to accommodate a total of 6.5 kW of PV roofing. There are 30 120-Watt laminates on one building, and the other two each have 24 60-Watt laminates bonded to architectural standing seam metal roofing pans. The line-tie net metered system uses two 4 kW true sinewave inverters which produce 220 Volt AC that feeds directly into the utility grid. The system qualified for both the CEC and the UPVG buydown and cost $8.55 per Watt before buydown and $4.49 per Watt after the rebate.

 The four buildings in this project are designed to house 17 people. Normally, 17 people would require over 10 kW of PV power, but the modest size of this project allows the 6.5 kW of PV roofing to produce as much electricity as they need. This net metered system challenged the utility to comply with the 1999 commercial net metering law. The challenge was to do net metering on seven separate meters.

 The utility solved the problem by separately metering the output of the PV arrays and subtracting that output proportionally from the grid-supplied power to the seven units.

 Fig. 11: 6.5 kW roof on co-housing project

 8.15 Project No. 15

 This project included the reproofing of a barn and an adjoining house with 6.5 kW of PV roofing. The PV array consists 28 60-Watt laminates and 40 120-Watt laminates bonded to architectural standing seam metal roofing pans. The grid interactive system also includes a 1.2 kW micro hydro turbine and a 500-Watt wind generator. For purposes of calculation, the cost of the two 4 kW true sinewave inverters and the 80 kWh battery pack is divided between the micro hydro/wind generator and PV system. The system qualified for both the CEC and the UPVG buydowns. The cost before rebate is $7.40 per Watt and after the buydowns is $3.27 per Watt.

 Fig. 12: Control room for 6.5 kW PV array with 1.2 kW micro hydro and 0.5 wind generator

 Micro hydro and wind installations provide an excellent complement for a PV array because they produce energy during stormy weather when there is not as much solar energy available. This project had the lowest overall cost per Watt of any retail installation, which is partially due to the fact that the inverter’s cost was shared by three renewable energy sources and most of the system was owner installed.
 

9. CONCLUSION

 As these projects show, building integrated PV arrays are less expensive than rack-mounted crystalline or polycrystalline modules. But more importantly, building integration provides an aesthetically acceptable way of using solar energy to produce electricity.  Several conclusions can be drawn in reviewing the projects. Line-tie systems are generally less expensive than interactive or independent systems, which require batteries. The cost of the inverter and the battery pack are fixed expenses that drive up the cost per Watt for small installations. Larger systems generally have a lower cost per Watt.

 Currently available buydown programs can cut the cost of PV systems by 50% making them affordable for the average homeowner. There are few building materials, if any, that pay back their initial cost in any time frame. But with current incentive programs, the time it takes for a PV system to pay off its cost can drop below 10 years. With 20-year warranties on power production and life expectancies far exceeding that, PV installations will pay double or triple dividends over their lifetime and provide free, clean, renewable energy for decades. The price will drop further as production increases providing a much needed alternative to conventional electricity generation.
 

10. REFERENCES

(1) Swan, Christopher S., "SunCell: Energy, Economy & Photovoltaics", Sierra Club Books, San Francisco, CA 1986
(2) National Renewable Energy Laboratory, "Solar Electric Buildings: An Overview of Today’s Applications", DOE/GO-10096-253, Washington, DC. 1996
(3) Alan Chuzel, "Module Shoot-out", Solfest ’98, Golden Genesis: Photocomm Final Report, 7-30-98