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This article was authored by Michael M.D. Ross, principal of RER Renewable Energy Research, while he was an employee of the CANMET Energy Diversification Research Laboratory (now known as CETC-Varennes).
Parts of this article are based on work conducted under PV for the North, a five year photovoltaic research, development and technology transfer program supported by Natural Resources Canada and the Nunavut and Aurora Research Institutes.
The author wishes to acknowledge François Leblanc and Sarah Mihailovich, formerly of CEDRL, for their work developing and validating the above model.
Data for the validation of the model came from a joint project with PanCanadian Petroleum Ltd. and Soltek Solar Energy Ltd.
Michael Ross. "Estimating Wintertime Battery Temperature in Stand-Alone Photovoltaic Systems with Insulated Battery Enclosures". Renewable Energy Technologies in Cold Climates: Proceedings of the 24th Annual Conference of the Solar Energy Society of Canada, Montréal, Québec, Canada: May 4-6, 1998, pp. 344-350.
Storage battery performance is strongly related to temperature: when cold, a battery's capacity, efficiency, and ability to accept charge decline. In cold climates, the lead-acid batteries commonly used in remote stand-alone photovoltaic (PV) systems typically operate at low temperatures during the winter. In order to ensure that the system functions reliably, the battery must be sized to accommodate poor winter performance.
Designers and installers generally use simple computerized algorithms and simulations to size stand-alone photovoltaic systems. While most of these softwares contain reasonably sophisticated models for calculating insolation, module temperature, module output, controller operation, and battery function, very few incorporate any reasonable model for battery temperature. Many assume that the battery will be at ambient temperature, at a specified minimum temperature, or at the average temperature over the day. In certain cases, these assumptions are sufficient. With larger battery banks and insulated enclosures, however, they are largely inadequate. It is impossible that these software can accurately model system performance in these cases; in spite of this, sizing and simulation software often include estimates of loss-of-load probability to several significant figures.
Modelling battery temperature is relatively simple. Solution of a simple ordinary differential equation yields a model for battery temperature requiring as parameters the thermal conductivity of the enclosure wall, the enclosure shape and size, the battery mass, and the ambient air temperature. This model is reasonably accurate; higher accuracy can be obtained by accounting for the inefficiency of the battery, which results in heat dissipation within the enclosure. In addition, the model can be adapted for use with phase change materials (PCM), which can establish a temperature "floor", below which the battery temperature will not drop.
This model has been validated for several sites in the Calgary area. The sites were monitored for a period of one year; the model was found to be highly accurate during the winter. The model was less accurate during the summer, probably due to solar heating of the exterior of the enclosure. This experimental validation also demonstrated the technical feasibility of using water as a phase change material to moderate battery temperature.
The implications of the model are explored in two ways: first, the model is used to examine the performance of various enclosures, with and without insulation and phase change materials, and second, the paper demonstrates the importance of incorporating such a model into simulation and sizing software.
Created 2005/10/05 Updated 2007/03/08 ©2005-2007 RER Renewable Energy Research