Abstract
Seasonal storage of thermal energy, by pumping heated water through a borehole array in the summer, and reversing the water flow to extract heat in the winter, can ameliorate some of the intermittency of renewable energy sources. Simulation can be a valuable tool in enhancing the efficiency of such storage systems. This paper develops a simple, efficient mathematical model of spatial temperature dynamics that focuses on the radial water flow in a cylindrical borehole array. The model calculates the time course of the temperature difference between outgoing and incoming water accurately, and allows new optimization strategies to be explored easily. A strategy based on discharging water heated by the array before it reaches the array center can increase the storage capacity by 25% for a system with a 20% smaller radius than the well-studied Drake Landing system. If the density of boreholes is also doubled, the improvement is 29%.
Highlights
A major obstacle to extensive deployment of renewable energy sources is their intermittency, both daily and seasonal
Hot water from sources such as rooftop solar collectors is pumped into a short-term storage tank
Cs = 3.203 MJ/m3 ◦ C is taken from McDowell and Thornton [6], who obtained Cs by fitting to the temperature data from Drake Landing
Summary
A major obstacle to extensive deployment of renewable energy sources is their intermittency, both daily and seasonal. For seasonal storage to be economically feasible, the cost needs to be on the order of. $1 per kWh [1], about two orders of magnitude below current battery prices. Borehole thermal energy storage (BTES) [2,3,4] is much less expensive. Hot water from sources such as rooftop solar collectors is pumped into a short-term storage tank. Hot water from the tank is pumped into the center of an array of U-shaped pipes in boreholes (Figure 1a), typically with a uniform spacing. When the water leaves the borehole array, it is transported back to the storage tank to be reheated, and reenters the array
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