ABSTRACT

This paper describes a geographic evaluation of Zero Energy Home (ZEH) potential, specifically an assessment of residential roof-top solar electric photovoltaic (PV) performance around the United States and how energy produced would match up with very-efficient and super-efficient home designs. We performed annual simulations for 236 TMY2 data locations throughout the United States on two highly-efficient one-story 3-bedroom homes with a generic grid-tied solar electric 2kW PV system. These annual simulations show how potential annual solar electric power generation (kWh) and potential energy savings from PV power vary geographically around the U.S. giving the user in a specific region an indication of their expected PV system performance.

Procedure

Using the energy simulation software EnergyGauge USA ( EGUSA), we simulated annual PV power generation in all 236 TMY2 sites giving us clear information on how PV production varies throughout the U.S. In changing the TMY locations we applied utility rates to fit each particular state’s average utility costs for both natural gas and electric. We assumed natural gas for all low-grade thermal heating applications (space heat, hot water, cooking, dryer) as these end-uses are not thermodynamically appropriate for high cost solar electricity. Within the analysis, net metering was assumed so that revenues from PV generation were valued at the same rate as energy supplied by the utility. Although time-of-day pricing would likely make the PV look even more attractive in applicable regions, laws preventing net metering in some locations would make PV look less favorable.

Analysis spanning over two decades has shown that solar energy has greatest merit when applied to buildings, which have been made very energy efficient (e.g. Balcomb, 1980; Parker and Dunlop, 1994). More recently Zero Energy Home designs have demonstrated the potential for energy self-sufficient residences when very high levels of efficiency are matched with solar hot water and solar electric power production (Parker et al, 2000). Accordingly, two generic highly efficient homes were simulated in all locations to see how solar electric power production matched up with the building loads with the two progressively more efficient designs. This allows a geographic assessment of ZEH potential. Comparison with a standard highly efficient home shows the increasing value for efficiency.

Building Simulation Analysis

A detailed hourly building energy simulation, DOE 2.1E, was used to assess the hourly energy use and energy cost. DOE-2 predicts the hourly energy use and energy cost of buildings given hourly weather data, a detailed description of the building, its HVAC equipment and the prevailing utility rate structure (LBL, 1984). The utility cost rates where provided by the Energy Information Administration (EIA) , created by Congress in 1977, and are a statistical agency of the U.S. Department of Energy. These rates represent data from 2001 average price delivered to residential consumers by state.

The simulations were performed on an hourly time step with results compiled on an annual basis (8,760 hours). Typical Meteorological Year data (TMY2s) were used for all locations. A specifically enhanced implementation of the software, EnergyGauge USA was used for the analysis. This program has been validated in its predictions of cooling electric demand in three carefully characterized homes in Central, Florida (Fuerhlein, 2000).

Description and Comparison of Generic Efficient Homes

Two generic efficient homes were used for this analysis. One was designed as a highly efficient prototype and would represent current day best energy efficiency practice similar to that within the Building America Program (www.buildingamerica.gov). These prototypes were configured so they could be considered energy-efficient when moved throughout the U.S. Both buildings are similar in dimensions having 2,000 ft 2 of conditioned floor area with an attached garage. They differ in insulation values, cooling efficiencies, lighting characteristics, infiltration, tightness and water heating technologies with the Prototype ZEH as more efficient. Tables 1 and 2 summarize the key efficiency specifications for these two homes used for our analysis. Changes to the ZEH prototype are shown in bold typeface in Table 2.

Table 1. Building Specifications for Highly Efficient Prototype

Table 2. Changes to Building Specifications for Prototype Zero Energy Home

Utility Interactive Photovoltaic System

The photovoltaic (PV) solar electric generation system is a grid-interactive system producing DC current that is inverted into AC current and then directed to the local utility feeder. The PV generation system is a typically sized system with the aim to provide power that would offset much of household electrical loads. The PV Form (Menicucci and Fernandez, 1988) simulation model incorporated in EnergyGauge USA provided an estimate of the PV array electrical output. Based on the predicted loads for a peak day, a 185 sqft 2kW solar array was selected. The entire array would face south located on a roof at a 5/12 pitch (23 degrees) to favorably utilize solar radiation.

Siemens SP75 solar modules were selected for the evaluation. These single crystalline modules have a maximum power rating of 75W each making a total of 2025W for the system at standard operating conditions. A Trace U2512/24/32/36/48 2.5 kW AC power inverter was selected to convert the DC power from the array to alternating current. Table 3 summarizes key parameters for the PV and inverter data used in the PV Form simulations within EGUSA.

Table 3. PV and Inverter System Description

Hourly weather data used for the simulation was taken from the User’s Manual for TMY2s Typical Meteorological Years derived from the 1961-1990 National Solar Radiation Data Base (NSRDB). TMY2 is a data set of hourly values of solar radiation and meteorological elements for a one-year period. It consists of months statistically selected from individual years and concatenated to form a complete year. The intended use is for computer simulations of solar energy conversion systems and building systems (Marion and Urban 1995).

Simulation Results

We evaluated the data from the simulations in all TMY2s locations summarizing by city and state. Table 4 shows the combined total of all estimated annual loads for the ZEH in kWh and therms with PV listed in kWh of power produced. The PV offsets the electric costs by sending power back to the grid-interactive system. Estimated combined electric and gas costs are listed to show the effect of state-level average utility charges for fuels. Annual PV power produced (kWh) and savings are also listed by percent electric and percent total cost to show how much the PV contributes in offsetting energy loads and costs for each site.

Based on this analysis, an average of the calculated percent of total energy cost was taken for both simulated homes. The average percent of total energy cost provided by the PV system for all locations is 37 percent for the ZEH, but only 27 percent for the highly efficient home. Thus, making key efficiency improvements can significantly improve the fraction of energy that typical PV systems provide.

For the ZEH prototype the 2kW PV array produced 44-106 percent of electrical needs around the continental U.S. (average is 69 percent). Similar values for percent of total energy cost produced varied by 25-88 percent with an average of 39 percent. On a state-by state basis, the concept loads are particularly attractive in California with its low space conditioning loads and good solar availabilities.

Table 4. Summary of Simulation Results