Analysis and Testing of a Portable Thermal Battery

Abstract

Portable energy storage will be a key challenge if electric vehicles (EVs) become a large part of our future transportation system. A big barrier to market uptake for EVs is driving range. Range can be further limited if heating and air conditioning systems are powered by the EV's batteries. The use of electricity for HVAC can be minimized if a thermal storage system, a “thermal battery,” can be substituted as the energy source to provide sufficient cabin heating and cooling. The aim of this project was to model, design, and fabricate a low-cost, modular thermal battery for EVs. The constructed thermal battery employs a phase change material erythritol (a sugar alcohol commonly used as artificial sweetener) as the storage medium sealed in an insulated, stainless steel container. At a total prototype cost of ∼$311/kW-h, the system is roughly half the price of lithium ion batteries. Heat exchange to the thermal battery is accomplished via water (or low viscosity engine oil), which is pumped through a helical winding of copper tubing. A computational fluid dynamics (CFD) model was used to determine the geometry (winding radius and number of coils) and flow conditions necessary to create adequate heat transfer. Testing of the fabricated design indicates that the prototype thermal battery module can store enough heat and discharge it fast enough to meet the demand of cruising passenger vehicle for up to 1 h on a cold day. The battery is capable of storing nearly 100 W-h/kg and can provide a specific power density of 30 W/kg. The storage density is competitive with lithium ion batteries, but work is needed to improve the power density.