Research paper on Photovoltaic technology on building surfaces.

RESEARCH PAPER
ON

ENERGY CONSERVATION: THE APPLICATION OF SOLAR ENERGY PHOTOVOLTAIC TECHNOLOGY ON BUILDING SURFACES

Submitted by

Dipankar Roy
Bhargab Handique

(Department of Civil Engineering,
Bapurao Deshmukh College of Engineering, Sevagram)

SUMMARY:

Energy conservation and consumption patterns have been a serious concern over the past thirty years. With the new age of globalisation and changing patterns in human needs, the need for energy has raised manifold. With the demand of energy rising, the need for new sources of energy became the first priority. This gave birth to the idea of alternative green energy. This paper deals with the need for conservation of traditional energy sources and development of alternative ones, especially solar energy in this case. The paper deals with the concept of green buildings and the impact civil engineering can bring to the movement of energy conservation by integrating buildings with the photovoltaic technology. How such a technique can make the buildings self sustaining in case of energy demand and consumption over the years. A few case studies have also been involved in the paper along with the credibility of such integration when we look 30 years down the line. Over much of the past 30 years, the standard for solar technology installed on buildings has been rack-mounted, silicon crystalline-based solar panels. Today, a new type of technology called thin-film photovoltaic is ready to change the way solar collectors are installed on buildings. This paper gives a broad overview of both the old and the new solar technologies, plus insight on designing and installing photovoltaic systems.

CONTENTS:

Sr. No
Table of contents
Page No.
a.
Introduction
1
b.
Background / Methodology
2

History
2

Methodology
2

1. Photovoltaic (PV) basics
3

1.2 Crystalline silicon
4

1.3 Thin-film photovoltaics
4

1.4 Building Integrated photovoltaic (BIPV) systems
5

2. Roofing
5

2.1 Metal Roofs
6

2.2 Single-ply systems
6

2.3 Standard and Modified-Bitumen Roofs
7

2.4 Steep-slope systems
7

3. Wall and window systems
7

4. Cost
7
c.
Overall systems
9
d.
Conclusion
10

INTRODUCTION:

The concept of energy conservation has grown more than just a concept. The need to realize the conservation of energy has grown by infinite proportions and need serious contemplation. Simply conserving energy by altering our consumption patterns is not a solution. Especially not if we are looking to realize our long term commitments to energy conservation. The very aspect of energy conservation has now shifted to renewable energy sources. Few call it the alternative energy some call it non-conventional energy while the rest prefer to call it the green energy. Whatever is the case, understanding the need and long term benefits of renewable green energy can solve our energy problems and can help restore the movement of energy conservation. As mentioned, here in this paper we are going to look into various aspects and possibilities of integrating photovoltaic technology into building surfaces. In other words, to explore possibilities of the use of solar energy to help buildings become self supporting energy systems. The interest in photovoltaics is growing rapidly worldwide. One of the main focus areas in the introduction of photovoltaics as renewable energy power source is the use of building surfaces for photovoltaic installations. To support the development of sound market introduction policies for photovoltaics, it is valuable to have knowledge of achievable contributions of photovoltaics to renewable energy portfolios, given the availability of building surfaces. In order to assess the potential of building integrated photovoltaics (BIPV), an analysis of the building stock with respect to suitability of the building skin for deployment is required. The objectives of this study are with respect to (the determination of) the BIPV potential:
To assess and compare different approaches, potential estimates and case studies.
To formulate an accepted and validated methodology.
To develop a comprehensive set of rules of thumb.

BACKGROUND / METHODOLOGY:

History

Photovoltaic applications for buildings began appearing in the 1970s. Aluminum-framed photovoltaic modules were connected to, or mounted on, buildings that were usually in remote areas without access to an electric power grid. In the 1980s photovoltaic module add-ons to roofs began being demonstrated. These PV systems were usually installed on utility-grid-connected buildings in areas with centralized power stations. In the 1990s BIPV construction products specially designed to be integrated into a building envelope became commercially available. A 1998 doctoral thesis by Patrina Eiffert, entitled An Economic Assessment of BIPV, hypothesized that one day there would an economic value for trading Renewable Energy Credits (RECs). A 2011 economic assessment and brief overview of the history of BIPV by the U.S. National Renewable Energy Laboratory suggests that there may be significant technical challenges to overcome before the installed cost of BIPV is competitive with photovoltaic panels.

Methodology

The existing different approaches and data sets imply, of course, studies of lower, intermediate and higher accuracy. A number of potential studies have either rough assumptions or a poor data base and therefore low accuracy. This can be justified by hinting at the huge area potential and the fact that photovoltaics still experiences economic restrictions much more than technical ones. Nevertheless, it would be inappropriate to use the most specific data sets and methods taking into account the main aspects described in the approach used in the report. Accurate BIPV potential studies are part of the fundamental base to evaluate the market potential and their target and focus groups, to assist the photovoltaic industry and building sector (with respect to BIPV products), utilities, and energy policy makers and to provide information to planners and lawmakers. However, in the methodology presented in the report, PIPV potential calculations are based on ground floor area figures, which are transformed into roof and facade surface figures. The BIPV potential calculations are based on ground floor area figures, which are transformed into roof and facade surface figures. The BIPV potential can subsequently be calculated by applying factors for solar yield and architectural suitability to the gross roof and façade surfaces. Architectural and solar suitability are described as follows:
Architectural suitability includes corrections for limitation due to construction (HVAC installations, elevations, terraces, etc.), historical considerations, shading effects and use of the available surfaces for other purposes.
Solar suitability takes into account the relative amount of irradiation for the surfaces depending on their orientation, inclination and location as well as the potential performance of the photovoltaic system integrated in the building.

The solar-architectural suitability is expressed in relative terms and results in utilization factors.

Figure 1: Most significant terms and factors for the BIPV potential

The utilization factors reflect the BIPV potential in most significant relative terms. In order to extract absolute figures in square meters and kilowatt hours, the relative figures have to be combined with the building areas and the solar irradiation available.

Factors for moving from ground floor area to roof and façade surface area, as well as for solar and architectural suitability, can be derived by analyzing representative samples with a limited number of buildings and sections of a particular building stock, and then up-scaling the results to the overall building stock. This sampling demands some resources and is part of an accurate methodology, which has been applied in Switzerland to a number of cities and states.

Building-integrated photovoltaics (BIPV) are photovoltaic materials that are used to replace conventional building materials in parts of the building envelope such as the roof, skylights, or facades. They are increasingly being incorporated into the construction of new buildings as a principal or ancillary source of electrical power, although existing buildings may be retrofitted with similar technology. The advantage of integrated photovoltaics over more common non-integrated systems is that the initial cost can be offset by reducing the amount spent on building materials and labour that would normally be used to construct the part of the building that the BIPV modules replace. These advantages make BIPV one of the fastest growing segments of the photovoltaic industry.

1. Photovoltaic (PV) basics: Photovoltaics (PVs) are one of the most promising sustainable renewable energy technologies. Solar (PV) modules produce electricity on site, directly from the sun and with the least environmental harm. Solar has the smallest environmental impact of any of the renewable energy systems. PV modules are solid-state devices that simply make electricity from sunlight. Solar requires little maintenance, produces no pollution, and does not deplete any non-renewable fossil energy resources, such as oil, natural gas, and coal.

1.2 Crystalline silicon:

We are all familiar with the basic PV glass module mounted on racks or posts. Glass-encapsulated crystalline silicon is the most common type of solar module. Most rack-mounted PV modules consist of crystalline silicon, either as a single or as a polycrystalline wafer, to generate electricity.
Silicon-based PVs have the highest electrical output of any PV material per sq ft.
Typical power production is between 12 and 18 watts per sq ft.
Silicon wafer PVs are more expensive and require large amounts of energy.
Silicon modules cost, on average, $4.85 per watt.
Solid silicon PV modules represent 95% of all solar panels installed.

Other building system applications include semitransparent PV glass modules for windows and skylights using crystalline silicon. Several PV manufacturers have also integrated non-glass, polymer-surfaced crystalline modules into new roofing products. Leading silicon panel manufacturers include Suntech, ET Solar, SunPower, Schott Solar, Sharp Corporation, and Canon Inc.

Figure 2: Crystalline-silicon array on a Voorheesville, NY school.

1.3 Thin-Film Photovoltaics

A number of new PV technologies have begun to emerge in the marketplace. These newer technologies, called thin-film photovoltaics, include very thin layers of photovoltaically active material placed on glass, flexible metal, or plastic substrates. Flexible thin-film PV modules are made by depositing semiconductor materials on stainless steel foil or a plastic carrier and encapsulating them with a solar transparent plastic polymer. Semiconductor materials used in thin-film PVs include:
Amorphous silicon (a-Si): Uni-Solar, Powerfilm, and Xunlight
Copper indium gallium diselenide (CIGS): Miasolé, NanoSolar, and Global Solar
Cadmium telluride (CdTe): First Solar

Figure 3: Grand Valley State University displaying Thin-film photovoltaics.

Other new solar technologies in development include dye-sensitized solar cells using a dye-impregnated layer of titanium dioxide to generate electricity. Dye-sensitized solar cells are printed onto various polymer films with equipment resembling computer printers. Recently, new carbon-based (organic) solar modules have been developed. Thin-film PV materials are used in both glass-encapsulated and flexible-membrane solar modules.

1.4 Building integrated photovoltaic (BIPV) systems: Building Integrated Photovoltaics is a new building technology concept. BIPV involves integration of PV modules into the building envelope by incorporating them in conventional building products, such as the roof, windows, or walls. BIPV is both the building envelope surface and the building energy source.
BIPV advantages:

A building owner’s BIPV system is connected with the local utility grid, and, with net metering, it can export surplus energy to the utility for later use. The building becomes a distributor of its surplus power production. Net metering banks or deposits that energy with a utility company. The power storage system is essentially free.
Both the building owner and the utility benefit, as on-site solar power production is typically greatest during times of peak energy need for both buildings and power companies. The PV system reduces energy costs for the building owner, and the exported surplus solar energy provides additional power to the utility grid during the time of its greatest energy demand.
The utility company can maintain needed power production capacity without the capital investment of building new power generation plants.
The building owner can draw back the net-metered power at night, when electrical costs tend to be lower, reducing power cost.
Photovoltaic systems can become the building’s primary energy source in the event of a power failure, and energy can be stored onsite using batteries for emergency backup.

2. Roofing:

Placing rigid solar PV roof panels as a stand-alone or rooftop-equipment-mounted power module array has been around since the invention of solar PV cells. The creative idea of integrating thin-film PV modules into a roofing system dates back to the 1980s. Only recently, with the commercial development of newer, structurally flexible, PV thin-film and polymer-surfaced crystalline-silicon modules, have we seen the integration of PV with traditional roofing materials. Today, there are a number of new roofing systems and products on the market using laminated PV, flexible-film, and polymer-surfaced crystalline. These include shingles, roof tiles, metal roofs, modified bitumen, and single-ply roof membranes. These are dual-function products that weatherproof the building while generating renewable energy from the sun.

2.1 Metal Roofs:

Uni-Solar pioneered the application of flexible, thin-film PV modules to architectural metal roof panels. Its flexible, thin-film module has a pressure-sensitive, peel-and stick adhesive on its back surface and can be factory laminated to metal roof panels for new construction. The self-adhesive, flexible, thin-film PV modules can be applied to existing snap-lock and batten standing seam metal roofs if the metal panels have a flat profile between seams.

Another solar metal roof attachment system by SolarPower uses high-energy flexible magnets for attaching thin-film PV to metal roof panels, creating a re-deployable solar roof system. The thin, nitride-rubber composite magnets and thin-film module systems perform well in high-wind conditions and have the advantage of being transferable to another solar roof host site.

Flexible thin-film modules are one of the best options to affix a PV array to metal roofs. Rigid glass silicon PV modules work but they require some form of mechanical attachment. It is important to determine how the additional weight of heavier glass modules and rack attachment systems affects the building structure and metal roof system.

2.2 Single-ply systems: One of the most common low-slope solar roof systems today uses a factory-laminated, flexible, thin-film PV module to a single-ply membrane sheet. This concept was patented by Solar Integrated Technologies over 20 years ago. Working together, Uni-Solar, Sika-Sarna fill, and Solar Integrated Technologies (SIT) created the first single-ply BIPV PV roof system. PVC single ply is the membrane polymer of choice, while TPO can also be used.

Figure 4: Open energy modules and single-ply membrane systems.

2.3 Standard and Modified-Bitumen Roofs:

PV systems integrated with standard asphalt and modified-bitumen BIPV roofs have lagged behind in development. With summer surface operating temperatures approaching 180˚F, attaching PV modules directly onto an asphalt-based roof membrane with a lower softening point has proven to be difficult. SBS-modified membranes tend to have lower softening points compared to APP and standard 90-lb cap sheets. The difference in thermal expansion between the modules, adhesives, and asphalt roof makes direct adhesive attachment challenging. Adhesive attachment to granule surfaces can be problematic. As the surface temperature rises, granules may not stay attached to the membrane surface if the systems (module and membrane) have a large range of contraction and expansion over the day/night thermal cycle.

2.4 Steep-slope systems:

Glass solar modules have been used on steep-slope residential roofs for years. The flat solar panels are easy to install, requiring only a few roof penetrations to run the wiring. However, glass PV modules are considered by some to be an unattractive roof system. With an expected service of 30 years and a higher surface temperature, glass modules will outlast most roof surfaces, especially those with asphalt shingles. There are a number of new alternative PV roofing products that have incorporated active solar materials into conventional roof products to create true BIPV roofing products.

New solar shingle and tile products use either thin-film or crystalline silicon for a solar-active surface. A number of new solar tile modules that can interface with flat, concrete roof tiles are available. The Sun Power Sun Tile is a high-efficiency solar panel that blends invisibly into concrete-tile roofs. GE Solar makes a solar tile with a peak power of 55 watts at 8.4 volts and has a unique interlocking design for concrete-tile applications. SharpSolar offers polycrystalline silicon modules with 62 volts of output. Open Energy makes a polycrystalline, tile-type module that comes in brown, red, and black. Atlantis Energy Systems manufactures a slate-type tile that works well with flat tiles and slate roofs.

3. Wall and window systems:

There are a number of new wall and window PV systems. The key limiting factor with wall and window application is the building placement relative to the sun. South, southeast, and southwest walls produce the most power. Window systems are typically a silicon wafer or thin-film cell laminated between two sheets of glass and are semitransparent. Solar window applications include window glazing, curtain walls, atriums, and skylights. Currently, wall and window systems are very specialized. Keep watching as the pricing of solar systems continues to drop, and more building wall and window systems start to incorporate solar into more traditional wall and window building products.

4. Cost:

Specifying and selling PV roof systems is longer and more complex than that for standard roof systems. PV systems are expensive. The average installed cost of a commercial low-sloped PV roof system before tax credits and state incentives can average between $50 and $150 per sq ft, depending on the power density of the PV system. The solar industry uses cost-per-watt pricing. Tax credits and state incentives are priced as dollars per watt. The average cost per watt for solar arrays installed on low-sloped commercial applications is between $8 and $12 per watt DC, excluding the cost of the roof.

Governmental bodies and utility companies offer performance-based incentives, grants, government tax credits, and rebates to make solar affordable. Unfortunately, there is no uniform, state-to-state system of tax credits, incentives, and programs. For the next eight years, the federal government will provide a 30-percent tax credit and five-year accelerated depreciation on commercial and residential solar installations. Several states now mandate that a percentage of utility companies’ electrical power production is generated from solar and other renewable sources, and more utility companies are starting to offer solar incentives. Determining what financial options are available to the building owner is an important part of the solar design and sales package. After all, while a 300-square roof may cost $200,000, a 100-kW solar array before tax credits and incentives can easily cost $800,000 to $900,000.

OVERALL FINDINGS:

Some general statements can be derived concerning the solar electricity production potential:
Assuming good solar yield of about 80%, achievable levels (ratio * BIPV solar electricity production potential / current electricity consumption) of solar power production by photovoltaic roofs and facades vary from 15% to almost 60%. Applying some strict solar yield criterion of 90%, these achievable levels are almost reduced by a factor 2 (from 8% to 30%). If all the architecturally suitable building area is used, the achievable levels are nearly double (from 30% to almost 120%).
The BIPV solar electricity production potential is even larger when a more progressive global conversion efficiency rate (more than 10% solar electricity output out of total solar energy irradiated) is assumed.
The achievable levels depend (besides technical aspects) mainly on the building areas available and, obviously, on solar irradiation and the electricity consumption.
The achievable levels are significant higher for the US and Australia on one hand, and much lower for Japan on the other hand. This is mainly due to available building areas.
There is an average of 18 meter square roof area per capita potentially usable for photovoltaics in Central Western Europe. Whereas for US/Australia, this figure is approximately 36 square meters, whereas in Japan only 8 square meters are available per capita.
About 15%-20% of the BIPV electricity production potentially can be attributed to façade areas.
Interestingly, the relative share of solar-architecturally suitable area is fairly coherent within and between the countries considered, i.e. the utilisation factor is 0.4 for roofs and 0.15 for facades. This enables one to assess the BIPV potential with easy-to-use rules of thumb.

CONCLUSION

As an industry, solar is an area in which we need to become fully engaged. Looking back over the years, we can all remember some of the mistakes made when new roofing technologies were introduced and not properly studied or tested. Solar power and roofing are going to play an important role in creating a new national energy policy and energy resource. We need to get solar power and roofing right the first time. This in the long run not only provides stability in terms of energy independence but also provides a future to the concept of energy conservation.