According to a proposal for small electric-power systems that must operate in cold environments, heat released in the operation of solar photovoltaic arrays with solar concentrators would be utilized to maintain batteries at operating temperatures. The proposal is straightforward and does not impose a requirement for any fundamental new technological developments. The proposal is applicable to secondary batteries, which are charged by use of the photovoltaic arrays.
Common batteries that contain aqueous electrolytes perform poorly at temperatures below ~10°C, because the electrolytes freeze and/or become viscous, with consequent degradation of ion-transport properties. The low-temperature performances of some lithium primary and lithium-ion secondary batteries have been improved by use of nonaqueous electrolytes, but there remains a problem of what to do in environments colder than their lowest operational temperatures. The proposal offers an alternative solution for batteries of all types.
The typical energy-conversion efficiencies of modern solar photovoltaic cells range from 5 to 25 percent; consequently, most of the solar radiation focused by solar concentrators onto solar photovoltaic cells is converted to heat. It is necessary to remove excess solar heat because the efficiencies of solar photovoltaic cells decrease with increasing temperature. Heretofore, various forms of active and passive cooling have been used to remove heat from solar photovoltaic cells. The basic concept of the proposal is to divert some or all of this heat to the batteries instead of rejecting all of it to the environment.
The figure illustrates an example of how the proposal might be implemented in a system in which the solar concentrators are simple flat mirrors located between subarrays of solar photovoltaic cells. The batteries would be placed behind the subarrays. Preferred configuration might consist of thin type batteries coupled with photovoltaic cells which would be considered to be amenable to a wide range of applications. The batteries would be thermally connected to the subarrays via heat-transfer-management subsystems and layers of heat-conductive material. Depending upon the specific application, the heat-transfer-management subsystems in a given system might contain heat pipes, thermostats, and/or heat reservoirs containing phase-change materials. Phase-change materials could be used to reduce temperature fluctuations. Heat pipes could be used during periods of low insolation to transfer, to the batteries, heat stored previously during peak insolation. Thermostats could be used during peak insolation to break the thermal connections when temperatures exceed a specified value.
The innovations discussed here are formulated concepts, and have not been fully reduced to practice.
This work was done by Julian Blosiu, Marshall Smart, Garry Burdick, and Subbarao Surampudi of Caltech for NASA's Jet Propulsion Laboratory. NPO-20284