Design of a thermochemical heat storage system for tap water heating in the built environment

Replacing the use of fossil fuel by solar energy, as one of the most promising sustainable energy sources, is of high interest, because of climate change and depletion of fossil resources. However, to reach high solar fractions and to overcome the mismatch between supply and demand of solar heat, storage of solar energy is necessary. A reliable method for long term heat storage is to use thermochemical materials, TCMs. The heat storage process is based on a reversible adsorption-desorption reaction of water vapor on the TCM, which is exothermic in one direction and endothermic in the reverse direction. In this research, Zeolite 13X is used as TCM. The system is an open sorption heat storage system for providing hot tap water. In the experimental test setup, the humid air is provided in a bubble column by blowing air from bottom of the column. The exothermic hydration process starts with humid air entering into a packed bed reactor filled with zeolite 13X. The reactor is a vertical cylindrical tank which is made of steel; it has a layer of Teflon inside and has a layer of insulation outside. The temperature profile in the reactor is measured as a function of time both along the flow direction and perpendicular to the flow by thermocouples. In addition, input and output temperatures and humidity are measured. In the resulting adsorption reaction between water vapor and TCM, energy is released. This released energy heats up the air flow which passes through the reactor and the hot output air flow is used to heat up the water in a water tank. The water tank is also a vertical cylindrical tank which is made of steel and has a layer of insulation outside. The hot output air from the reactor passes through a coiled tubing inside the water tank to heat up water. The temperature of the water in the tank is measured at two different heights. A problem in open solid sorption systems using air as heat transport medium is the limited temperature step which can be achieved in the sorption bed. In the present study this problem is solved using a heat recovery system enabling higher output air temperatures. The residual heat in the exhaust air is used to preheat the reactor inflow, in an air-to-air heat exchanger. In the endothermic dehydration process, the hydrated zeolite is dried with hot air. In this study, a lab-scale prototype TCM based heat storage system is designed and optimized, which, by making use of a heat recovery loop, is able to provide hot tap water. Results of the experimental investigation on charge-discharge cycles will be presented.