Energy storage for air-conditioning using CO2 gas hydrates

Abstract

The prevalence of air conditioning in buildings and its synchronicity within a geographic region causes peak load capacity problems for electricity transmission and distribution infrastructure. While solar photovoltaics can assist in mitigating midday and early afternoon peak loads, late afternoon and evening loads cannot be serviced in this way. Presently, battery storage is not as cost effective as thermal storage for air conditioning. However, thermal storage systems are not widely adopted in this field. In this paper, we discuss a range of issues associated with the use of phase change energy storage at typical air conditioning evaporator temperatures. These issues include sub-cooling, melt-freeze hysteresis, thermal conductivity, energy density, cost and cyclic stability. We present a phase change material tailored for storage of energy at a range of temperatures between 5°C and 12°C. The material is a type of gas hydrate formed from water, a salt and carbon dioxide gas. The energy store operates in a pressurized heat exchanger whereby the phase change temperature may be tuned by modifying the vessel pressure over a range of 4-8 bar. The gas hydrate has appearance and properties similar to ice. Through systematic development, the material now exhibits an energy storage density of around 90 kWh/t, similar to conventional ice and superior properties to most other phase change materials. Additionally, the gas hydrate may be used with conventional chillers operating at conventional evaporating temperatures. The cost of the energy storage system, including material and heat exchanger is estimated to be around $250/kWh at lab scale. The main field of application of the gas hydrate cold store material is to allow air conditioning chillers to run overnight, storing the cooling effect for deployment during the following day. Not only can the chiller operate on low cost off-peak electricity, but also the chiller runs more efficiently when heat is rejected to cooler overnight temperatures. The next steps involve scale up to commercial air conditioning installations in the cooling capacity range 10-100 kW. The gas hydrate store would run in parallel with the chiller on a heat exchange loop, thereby providing fail-safe operation. Show more