Seasonal solar energy storage using thermochemical adsorption for space heating

Abstract

Adsorption-based thermochemical heat storage is a long-term thermal energy storage technology that can be used for seasonal solar energy storage and especially for space heating, which has received significant attention. Pure salt as the adsorbent, even though has huge water-adsorption capacity, takes the disadvantage of low thermal conductivity, deliquescence, and swallow problems during cyclic sorption operations, which could finally reduce both the thermal energy storage and the space heating performance. Composite adsorbent, integrating the salt with the porous matrix such as activated carbon and zeolite, could keep the high water-adsorption capacity (energy density) when the porous structure of the matrix providing a more stable form to hold the adsorbed/desorbed adsorbent and to prevent it from agglomeration.
In this project, an efficient adsorption-integrated space heating system has been investigated with the seasonal solar energy storage (SSES) function using the composite adsorbent made by the salt and the zeolite. The water-adsorption space heating system using the composite which could take the advantage like that the water vapour widely spreads in the environment and the zeolite is a popular material for the concrete mixing in the sustainable architecture research.
The composite adsorbent made by salt solutions (MgSO4, LiCl, and LiBr) with different concentrations and various zeolites have been investigated as these salts are easy to obtain and safe for the environment. Moreover, beside the single salt-zeolite composite of MgSO4-zeolite, LiCl and LiBr could form the complex salt(A)-salt(B)-zeolite as LiCl-LiBr-zeolite composite to potentially increase the water-adsorption capacity and thus enhance its space heating and energy storage performance. The manufacturing methodology has been developed according to the practical experimental experience by the author. The prepared composite adsorbent has been experimentally characterized in respect of the surface morphology, thermal-physical properties, porous properties, and adsorption-related characteristics. The involved investigations include Scanning Electron Microscope (SEM) – Energy Dispersive Spectroscope (EDS), Nitrogen Adsorption, X-ray Diffractometer (XRD), and Thermogravimetric Analysis (TGA) - Differential Scanning Calorimetry (DSC). Besides, the adsorption kinetics of the composite adsorbent has been tested by the climate chamber, which could control the adsorption environment of the temperature and the relative humidity. For MgSO4-zeolite, the maximum heat storage density found in the TGA-DSC desorption tests was 481.3 J/g by MgSO4(20%)-4A, while the maximum water adsorption capacity obtained was 0.1803 g/g by MgSO4(20%)-13X. For LiCl/LiBr-zeolite, the maximum heat storage density, 592.8 J/g, is found in 5%LiCl-5%LiBr-zeolite. The most water adsorption capacity observed in the climatic chamber adsorption experiment is 0.22 g/g (5%LiCl-25%LiBr-zeolite).
As LiCl-LiBr-zeolite has proved its superior water-adsorption capacity as well as the energy density, they are selected as the adsorption composite to be used in the further space heating experiments. An adsorption pipeline reactor has been established to experimentally investigate the air heating effect by using the selected candidates. Moreover, a 1:22.5 scaled house model was printed by a 3D-printer to simulate the condition that the house model is heated by the adsorption heat provided by the tube reactor. The effects of air velocity and relative humidity on the adsorption performance has been examined. The outlet air and room temperature profiles are recorded for analysis. It is found that a flow rate of 15 m3/h and a relative humidity of 70% could lead to the maximum adsorption heat, 434.4 J/g, from the water-adsorption reaction by the composite material, and the highest energy discharge efficiency of 74.3%.
Furthermore, the adsorption-integrated space heating system has been established using the software of TRNSYS. The novel space heating model could collect and store the solar energy under sufficient solar radiation and utilize it to heat the house in the cold seasons. Simulated houses are assumed to be one in Newcastle, UK, and one in Urumqi, China. The gross heat supply by the adsorption-integrated space heating system in Urumqi and in Newcastle is 8862.68 kWh and 6466.39 kWh for the whole space heating seasons, respectively. The results demonstrates that the required room temperature is well kept by the designed space heating system using the composite adsorbent. The system could store the solar thermal energy from 0% of its full storage capacity to 100% during the seasons that the space heating is not required. However, at the end of each space heating season, the heat storage remains 42.85% and 57.00% in Urumqi and Newcastle, respectively. Because after the heat storage capacity is less than 95% of the full capacity, the storage module starts to work simultaneously with the space heating module. When the heat consumption is larger than the heat storage, the storage capacity percentage decreases. After a time, when the heat consumption is less than the heat storage, the storage capacity percentage increases until the end of the space heating season. Thus, the heat storage remains rather than running out. The value of the storage capacity percentage is dependent on the heat consumption and storage relationships.The extra solar radiation after full storage may be utilized into other functions. Moreover, the COP of the adsorption-integrated space heating system in Newcastle is 20% higher than that in Urumqi.
In a summary, using the composite adsorbent, the adsorption-integrated space heating system is promising to also achieve a high-efficiency and low-environmental impact seasonal solar thermal energy storage system.