Development and Analysis Tool for Photovoltaic-Powered Solar Water Heating Systems

Abstract

A photovoltaic-powered solar domestic hot water (PV-SDHW) system, patented in 1994, uses PV cells to generate electrical energy which is dissipated in several resistive heating elements within a water storage tank. The system incorporates a microprocessor controller to select the appropriate combination of resistors to cause the PV array to operate near maximum power during diurnal solar irradiation fluctuations. Two prototype installations are currently operating, one at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD and the other at the Florida Solar Energy Center (FSEC) in Cocoa, FL. Both prototypes have a dual-tank design and are instrumented to record all pertinent data on system performance and environmental conditions. Models of the NIST and FSEC installations have been constructed with the TRNSYS program. The operation of the NIST and FSEC systems has been simulated for the periods for which data were available. TRNSYS has proven an ideal means of simulating PV-SDHW systems as verified by comparison of modeled performance results with measured data. The verified PV-SDHW system model was extended to construct a generalized system design and analysis tool. A method was developed using the EES program to allow automated optimization of the values of the multiple resistive elements and their connection control strategy. This selection was found to be negligibly sensitive to geographical location and dependent only on the operating characteristics of the PV array. TRNSYS simulation models of three PV hot water system configurations were formulated with user-friendly front-ends to facilitate their use by utility representatives, manufacturers, and other persons interested in such systems. The models allow the user to specify many design conditions such as geographical location, hot water storage tank sizes, PV array size and configuration, multiple resistive element values, and hot water demand. The models provide detailed output for many system variables as well as results relating to the life cycle economic value of the installation from the home owner's point of view. Finally, by use of the EUSESIA software, means are provided to estimate the effect of a large number of PV-SDHW systems on a local utility in terms of electrical demand and air pollution reduction. This thesis describes the models developed, presents and discusses comparisons of measured and TRNSYS-predicted performance results, and discusses issues related to the modeling of PV-SDHW systems. The performance of PV-SDHW systems is compared to that of more conventional thermal SDHW systems. The PV-SDHW design offers many potential advantages over thermal systems in the areas of installation simplicity and system reliability. However, the cost, and to lesser extent the inefficiency, of photovoltaic devices prohibit cost- effective construction of PV-SDHW systems today. A case study in the design and analysis of a PV-SDHW system is presented for a family of four in Milwaukee, WI. While a thermal SDHW system sized to meet the needs of the family in Milwaukee could be purchased and installed for around $4,000, a comparable PV-SDHW system was found to cost nearly $20,000. Furthermore, the PV array of a PV-SDHW system would likely occupy a surface area more than three times that of the collectors of a thermal system of similar performance. A decline in PV module costs from their current $6.00 per peak Watt to about $1.00/W was found necessary to make the PV-SDHW system break even as an investment. A PV cost of $0.65/W was found necessary to make PV- SDHW economically competitive with thermal SDHW. The U.S. Department of Energy long-term goal is $0.50/W. The impact of a large-scale installation of PV-SDHW systems on the local Wisconsin utility was estimated using EUSESIA. A single PV-SDHW system might be expected to reduce utility peak power demand by 0.77 kW, energy by 4138 kWh per year, CO2 emissions by 5000 lbm per year, SO2 emissions by 37 lbm per year, NOx by 24 lbm per year, and particulate matter by 2 lbm per year. A program to install 5000 systems in its service area could provide a net savings of about $500,000 per year to the utility over the life of the systems.