Pumped Thermal Energy Storage

Abstract

At present, there is a worldwide awareness and concern about how global warming could impact daily lives and the need to switch to renewable sources of energy. Commercially, various green energy technologies such as hydro, wind, solar, nuclear fission, etc. are being used to harness their stored energy and convert it into electricity. However, amongst these, the natural sources of energy are limited in the operation by their intermittent nature. This potentially leads to the instability of the power grid system. Hence, it is difficult to replace the existing conventional fossil-fired energy generation systems with their renewable counterparts, unless and until, flexibility is introduced to damp the intermittency
of renewable sources of energy.

A Pumped Thermal Energy Storage (PTES) system, which uses a combination of a heat pump and heat engine to store electricity in form of heat and convert the heat back to electricity, could efficiently provide intraday to multiday flexibility. However, the literature reveals that this technology is not mature enough and requires enhanced research and experimental work.

The scope of this thesis includes a conceptual design, steady-state modelling, and optimization of a waste heat integrated PTES, alternatively called Compressed Heat Energy Storage (CHEST) system. Two configurations of the CHEST system were analysed: (i) CHEST with latent heat storage, and (ii) CHEST with sensible heat storage. A combination of R1233zD as a working fluid and sunflower oil
as a sensible thermal energy storage medium was selected and a Phase Change Material (PCM) as a latent heat storage medium. Towards this end, a sensitivity analysis is performed to examine the effect of various design parameters on the performance of the system. It is found that CHEST systems are not suitable for high-temperature storage applications (> 200°𝐶). Further, the effect of superheating in a heat pump, and the effect of waste heat source temperature on system performance is investigated.

An optimization study is conducted on CHEST with a sensible storage configuration to use a single heat exchanger for the charging and discharging cycle during sensible heat transfer with a storage medium. A
detailed comparison between the two configurations have been performed. Most notably a trade-off has to be set between the performance and economic viability of the system.

Finally, the integration of the optimised sensible heat storage configuration with a solar-powered alkaline electrolyser is evaluated. A 30 MW of heat from the electrolyser unit is considered to be recovered with the help of the CHEST system. Towards the end, a preliminary design of the CHEST components (heat exchangers and turbomachinery) is provided to estimate the overall sizing and performance characteristics of the components.