Research Publications

The photovoltaic (PV) system on the roof of the School of Natural Resources and Environment (SNRE) serves as another model for the installation of PV systems in urbanized areas, which could play a major role in energy self sufficiency and security while solving the nation's greenhouse gas emission problem.

The goals of the project were (1) to provide renewable electricity to the School of Natural Resources and Environment; (2) to serve as a demonstration site for two alternative PV technologies - amorphous and multi-crystalline - in a relatively large installation, which also includes a 30 kW inverter; (3) to support research and provide historical system performance data.

Among the potential benefits of PV in substitution for fossil fuel based electricity are the reduction of greenhouse gas emissions which impact the global climate; the reduction of criteria pollutant emissions such as nitrogen oxides (NOx), sulfur dioxide (SO2), and carbon monoxide (CO) which cause health problems and damage the environment; and finally, the reduction of toxic emissions which cause human and ecological harm.

No electricity generation is free from harmful air emissions and the comparison and quantification of potential environmental benefits attained from the use of renewable energy systems requires the application of life cycle assessment (LCA). The production of the PV modules also consumes energy and materials which are also responsible for the release of harmful substances. In this report, we use a LCA methodology to weigh the benefits of replacing electricity from the grid and from the central campus power plant against the environmental releases that result from manufacture of the PV modules.

The report evaluates a 33 kilowatt peak (kWp) PV system which is comprised of 88 KC120 multi-crystalline modules manufactured by Kyocera rated at 120 watts of peak power (Wp) each, 132 PV laminates (PVL136) rated at 136 Wp each, and 75 PVL62 rated at 62 Wp each. The laminates are manufactured by UNI-SOLAR using thin film technology. The total power of the KC120 array corresponds to 10,560 Wp, the total power of the PVL136 array corresponds to 17,952 Wp, and the total power of the PVL62 array corresponds to 4,650 Wp. Therefore, the KC120, PVL136, and PVL62 are responsible, respectively, for 32%, 54%, and 14% of the total power output of the PV system. Although the total power of the system sums to 33kW, the maximum power output is limited by the capacity of the inverter which corresponds to 30kW.

A comprehensive model was developed to evaluate the energy and environmental performance of the PV system. The LCA characterizes manufacture and installation of the amorphous and multi-crystalline modules, manufacture of the inverter and all the ancillary material that comprises the balance of the system (BOS), and includes: hardware, cables, combiner boxes, and mounting structures.

The prediction of the life cycle electrical output of the PV system takes into account the local available solar radiation, the conversion efficiency of the modules, and their position. The model created to determine the lifetime electrical output of the system is based on the Bird Clear Sky Model from the National Renewable Energy Laboratory (NREL). According to results from this model, 44,848 kWh are produced by the entire system per year. The lifetime of the system is assumed to be 20 years, which corresponds to the warranty of the amorphous modules.

The results of the LCA of the PV system are compared to the emissions of greenhouse gas, criteria pollutants, and toxic emissions from the electricity generation technologies that provide electricity to SNRE. Therefore, this study also includes an inventory of the emissions from the natural gas fueled campus power plant (CPP) and the regional electricity grid. Several impact categories are assessed in this report (Table 1).

One metric widely applied in assessing electricity generation technologies is the net energy ratio (NER). Figure 1 shows the NER of the three modules used in the system and the system average value based on a 20-year period of analysis. Calculation of the system's NER takes into account energy and material input in manufacture of the BOS components and the inverter. In the case of the NER for single modules, the manufacturing energy input for the inverter was allocated based on the share of electricity output for each array.

The NER compares the primary energy input over the life cycle of the electricity generation system to its electrical energy output. When NER is applied to renewable energy sources it also represents the leveraging capacity of the source because it computes the amount of renewable energy obtained through the consumption of fossil fuels, which accounts for the majority of the energy inputs in the production of the renewable energy system. The higher the NER, the greater is the fossil fuel leveraging capacity of the energy system. In contrast to the NER, the energy payback time (E-PBT) measures the time period required for an electricity generation system to offset the amount of primary energy input in the system, which usually comes from non renewable sources. The E-PBT of the system is based on the same assumptions regarding the BOS and the inverter's energy and material inputs.

Figure 2 shows the energy payback time for different arrays of the system and the system's average E-PBT. The E-PBT of the KC120 is higher because of its higher energy intensity during manufacturing compared to the PVL modules, and due to the need of an aluminum supporting structure for the modules. The KC120 modules are installed with the same tilt angle as the PVL modules on the standing seam for comparison purposes. However, because they require a metallic mounting structure they could have been positioned at a tilt angle that would increase their electrical output. We estimate that 15% more power could be produced if the modules are positioned at an angle that corresponds to the local latitude (42ºN). This would make the KC120 array more competitive with the PVL array, which currently lacks the capability of being adjusted at an optimal angle to maximize the incoming solar radiation.

In addition to leveraging fossil resources, the PV system avoids the environmental and health impacts associated with fossil fueled power plants. However, because part of the PV system output substitutes for electricity produced at a combined heat and power (CHP) facility, which also delivers steam to the buildings, the installation of the PV system would demand a compensatory system to supply the same level of service. The quantification of the avoided emissions is based on emissions released by the systems that traditionally supply electricity to the building less emissions due to manufacture and installation of the PV system and emissions due to the operation of a hypothetical  compensatory system.

The energy mix originally supplied to the building consists of 67% of electricity generated at the Campus Power Plant (CPP) and 33% of energy from the East Central Area Reliability Coordination Agreement (ECAR) electrical grid, which contains 90% coal and 10% nuclear energy. For each kWh generated by the CPP (CHP plant), 13.4 ft³ of natural gas are consumed.

The modeling of the environmental performance of the PV system is based on two different scenarios. The "Net Emissions" scenario assumes that the PV system substitutes for the actual energy supplied to the Dana Building, consisting of electricity and steam produced at the CPP and electricity delivered by the ECAR grid. This scenario assumes the use of a compensatory system to produce steam, which consists of a boiler with the same characteristics of the CPP but at a much smaller scale. The 'Net Emissions' scenario assumes that the PV system substitutes for the electricity provided by the ECAR grid, and does not take into account the compensatory system.

Several compounds, released during the extraction, transportation, and combustion of fossil fuels are highly toxic and carcinogenic. The potential impact of those compounds can be represented using the human toxicity potential (HTP) method, which renders the aggregated potential reduction of human cancer due to the substitution of electricity from the PV system for electricity from fossil fueled plants (Figure 3).

The reduction of toxic releases by the PV system due to the displacement of fossil fuels also provides significant benefits to the global environment. The cumulative greenhouse emission reductions for 3 different time horizons due to the installation of the PV system are reported in Figure 4 as metric tons of CO2 equivalent, which are calculated based on 100 year global warming potentials (GWP).

Finally, the project's outreach effort conveys the benefits of PV renewable energy use to both students and the general public visiting SNRE. A data acquisition system (DAS) installed in the building monitors the real-time output of the PV system and each one of its arrays. The DAS calculates the cumulative air emissions avoided such as: carbon dioxide, sulfur dioxide, nitrogen oxides, and mercury. It also calculates resource consumption savings such as: coal, uranium, and natural gas. The DAS also compares the current output of the PV system to the corresponding energy load in the building, and tracks the best hour for weekdays and weekend days in which the highest share of electricity consumed in the building was met by the PV system.

In summary, installation of the PV system brings about valuable human health and environmental benefits compared to nonrenewable technologies available for supplying power to the Dana building. It is undoubtedly a significant part of the greening of the Dana building, which can serve as an example for the University and the City of Ann Arbor. On a sunny day in mid-May 2005 the PV system met 23% of the power demand of the Dana Building. The pursuit of higher shares of power coming from the system can encourage energy conservation in the building.