Investigation of innovative thermochemical energy storage processes and materials for building applications

In this study, it is aimed to develop an innovative thermochemical energy storage system through material, reactor and process based investigations for building space heating applications. The developed system could be integrated with solar thermal collectors, photovoltaic panels or heat pumps to store any excess energy in the form of heat for later use. Thereby, it is proposed to address the problem of high operational costs and CO2 emissions released by currently used fossil fuel based heating systems in buildings. The aim of the study has been achieved by investigating and evaluating five of the following aspects: • Investigation of the feasibility of building integrated solar driven THS system under cold and mild climates, • Synthesis, characterization and physical experimentation of novel composite sorption energy storage materials • Development and investigation of a modular laboratory scale sorption reactor that use embedded air diffusers inside the sorbent for improving the energy storage density • Development and investigation of a full- scale modular solar driven THS system • Development and investigation of a heat pump driven sorption storage heater using multi-layer fixed bed sorption reactor These works have been assessed by means of computer simulation, laboratory and field experimental work and have been demonstrated adequately. The key findings from the study confirm the potential of the examined technology. Initially, a comprehensive review on thermal energy storage, with the aim of investigating the latest advancements on THS systems was performed. A comparative analysis on applicability of different heat storage methods for short term and seasonal heat storage under climate conditions in the UK, was also carried out. Results showed that short term heat storage is not a feasible option in the UK due to the very limited solar radiation. For the case of seasonal heat storage, it was found that, each 1 m3 of THS can provide averagely 14% of monthly (October to March) heating demand of a 106 m2 building, whereas LHS and SHS can provide 6% and 2% respectively. Later on, a range of candidate composite sorption materials were synthesized and characterized. Based on the applied characterization techniques, it was found that Vermicuilite-CaCl2 (SIM-3a) has excellent Ed coupled with good EMC and temc with its TGA analysis also suggesting significant mass loss in the working range 30 < T < 140 °C. Physical experimentation of the developed materials in a small scale custom test rig was also performed and in accordance with the characterization results, SIM-3a displayed the best hygrothermal and cyclic performance. These findings suggested that SIM-3a has very good potential for use in an open THS system. Upon completion of the material based studies, a 3kWh laboratory scale novel reactor using perforated pipes embedded inside the heat storage material was developed. The overall energy density of the reactor using SIM-3a was found 290 kWh/m3. Based on the obtained encouraging results, same concept was up scaled to a modular 25 kWh sorption pipe heat storage and similar energy density was achieved. Following the experimental work, theoretical analysis of the THS potential in Mediterranean climate conditions is conducted with a case study of the Island of Cyprus. The analysis results showed that the required heat storage volume to fully compensate heating demand of a domestic building in winter (December to February) is 5.25 m3 whilst the time required for charging the THS material with 8 m2 solar air collectors is slightly more than a month. The economic and environmental analyses results showed that payback period of the solar driven THS is 6 years whilst total CO2 emissions savings over 25 years lifetime is 47.9 tonnes. In order to validate the applicability of THS in Cyprus, a small prototype of integrated sorption pipe-solar concentrator was also developed and tested for room heating. It was found that adsorbent could be regenerated with solar energy during winter day time to be utilized at night for space heating. Study results also showed that sorption pipe with a heat storage volume of 0.017 m3 could meet up to 87% of the daily heat demand of a 12.4 m2 building. In order to validate the performance of the laboratory tested THS material and concept, a real scale (1000 kWh) modular solar driven THS system was developed based on the interpretation of the obtained theoretical, numerical and experimental data in earlier stages of the study. The preliminary testing on the prototype showed that each of four reactors could discharge a total of 248 kWh of thermal energy with an average thermal power of 4.8 kW. Additionally it is found that, in direct solar heating mode, transpired solar collectors used in the system could also generate daily total of 17 kWh thermal energy for the average solar intensity of 0.3 kW/m2. In the final stage of the study, a heat pump driven sorption storage heater was developed and investigated. The developed system performance was assessed with 5 different adsorption materials and under different operating conditions. The study results showed that Sim-3a and Vermiculite–(LiCl-CaCl2) (Sim-3cl) has the best hygrothermal performances and hygro-cyclic efficiencies. According to study results, COPs varies in the range of 1→2 depending on sorption materials properties and system operating conditions

Salt impregnated desiccant matrices for ‘open’ thermochemical energy conversion and storage: improving energy density utilisation through hygrodynamic & thermodynamic reactor design

In this study, the performance of three nano-composite energy storage absorbents; Vermiculite-CaCl2 (SIM-3a), Vermiculite-CaCl2-LiNO3 (SIM-3f), and the desiccant Zeolite 13X were experimentally investigated for suitability to domestic scale thermal energy storage. A novel 3 kWh open thermochemical reactor consisting of new meshed tube air diffusers was built to experimentally examine performance. The results were compared to those obtained using a previously developed flatbed experimental reactor. SIM-3a has the best cyclic behaviour and thermal performance. It was found that 0.01 m3 of SIM-3a can provide an average temperature lift of room air, ΔT = 20 °C over 180 min whereas for SIM-3f, ΔT < 15 °C was achieved. Zeolite provided high sorption heat in close approximation with SIM-3a, however, the higher desorption temperature requirements coupled with poor cyclic ability remain as obstacles to the roll out this material commercially. The study results clearly show that the concept of using perforated tubes embedded inside the heat storage material significantly improves performance by enhancing the contact surface area between air → absorbent whilst increasing vapour diffusion. The results suggest a linear correlation between thermal performance and moisture uptake, ΔT–Δw. Determining these operating lines will prove useful for predicting achievable temperature lift and also for effective design and control of thermochemical heat storage systems

Future cities and environmental sustainability

Massive growth is threatening the sustainability of cities and the quality of city life. Mass urbanisation can lead to social instability, undermining the capacity of cities to be environmentally sustainable and economically successful. A new model of sustainability is needed, including greater incentives to save energy, reduce consumption and protect the environment while also increasing levels of citizen wellbeing. Cities of the future should be a socially diverse environment where economic and social activities overlap and where communities are focused around neighbourhoods. They must be developed or adapted to enable their citizens to be socioeconomically creative and productive. Recent developments provide hope that such challenges can be tackled. This review describes the exciting innovations already being introduced in cities as well as those which could become reality in the near future

Theoretical analysis of the potential for thermochemical heat storage under Mediterranean climate conditions: Northern Cyprus Case

Thermal energy storage systems are gaining attention in recent years as they are now seen as one of the most promising solutions in order to increase utilisation of solar energy and reduce greenhouse gas emissions. On the other hand in the last decade, thermochemical versions of these systems have been widely researched for ‘seasonal’ storage of solar energy as they have the potential to store heat at ambient temperatures for extended periods of time without any degradation or heat loss. In this study a theoretical analysis of the thermochemical heat storage potential in Mediterranean climate conditions is conducted. A theoretical building located in the northern part of the island of Cyprus is considered as a case study and analysis done using real building data from a site located on the west region (Morphou) of North Cyprus. The analysis results showed that the required heat storage volume to fully compensate heating demand of the building in winter (December to February) is 8 m3 whilst the time required for charging the THS material (Vermiculite-CaCl2) with 8 m2 solar air collectors is slightly more than a month ('i.e.' 35 days during May and June) An analysis of thermochemical heat storage’s economical and greenhouse gas savings compared against gas heaters, electrical heaters and air sourced heat pumps, which are the popular methods for space heating in North Cyprus, is also presented. It was found that payback period of the thermochemical heat storage is 6 years whilst total CO2 emissions savings over 25 years life is 47.9 tonnes

A novel evaporative cooling system with a polymer hollow fibre spindle

A polymer hollow fibre evaporative cooling system with a novel configuration of fibre bundle is proposed. With the aim to avoid the flow channelling or shielding of adjacent fibres the fibres inside each bundle were made into a spindle shape to maximize contact between the air stream and the fibres. For the porous wall of hollow fibre, the vapour of evaporated water can permeate through it effectively, while the liquid water droplets can be prevented from mixing with the processed air. For various dry bulb temperatures (27 °C, 30 °C, 33 °C, 36 °C and 39 °C) and relative humidity (23%, 32% and 40%) of the inlet air, the cooling performances of the proposed novel evaporative cooling system were experimentally investigated. The variations of outlet air dry bulb temperature, wet bulb effectiveness, dew point effectiveness and cooling capacity with respect to different incoming air dry bulb temperature were studied. The effects of various incoming air Reynolds number on the heat and mass transfer coefficients, heat flux and mass flux across the polymer hollow fibre module were analysed. Experimentally derived non-dimensional heat and mass transfer correlations were compared with other correlations from literature. Due to the proposed spindle shape of hollow fibre bundle, the shielding between adjacent fibres could be mitigated greatly, therefore the heat and mass transfer performance of the proposed system demonstrated significant improvement compared with other designs reported in literature

Overview of working fluids and sustainable heating, cooling and power generation technologies

Dependency on energy is much higher than the past and it is clear that energy is vital for a sustainable and safer future. Therefore, urgent solutions are required not only to increase share of renewable resources but also more efficient usage of fossil fuels. This could be achieved with innovative power, air conditioning and refrigeration cycles utilising ‘long-term sustainable’ (LTS) fluids, especially air, water and CO2. In the article we provide a rational approach to the future use of working fluids based on our interpretation of the available technical evidence. We consider it self-evident that volatile fluids will continue to play major roles in cooling and power generation, however, new technologies will be needed that optimise energy efficiency and safety with minimum environmental impact. Concordantly we discuss the past and current situation of volatile fluids and present four innovative technologies using air/water cycles. Study results showed that there is a rapid development in heating, cooling and power generation technologies those use water/air as working fluid. These technologies demonstrate a potential to replace conventional systems, thereby to contribute to global sustainability in near future. However, further development on LTS fluids and materials also process intensification and cost reduction are vital parameters for future advancement of these technologies

Numerical and experimental analysis of a novel heat pump driven sorption storage heater

This study investigates a hybrid “solid sorption heat storage/air sourced heat pump” system for energy efficient heating of buildings. The proposed system could convert excess energy generated using photovoltaic panels/off-peak electricity to heat and charge the sorption material to store that heat for later use. The novel heat recovery process employed in the system enables high heat storage efficiency through condensation of desorbed moisture in a heat storage charging cycle. In this study five different sorbents were tested in a novel prototype system. Four sorbents were salt based composites (SIM’s) and one was Zeolite 13X. According to the results, the coefficient of performance (COP) of the system varied in the range of 1–2 for short-term operation (where t < 240 min) depending on the sorption material properties and system operating conditions. The overall performance of the prototype sorption storage heater was determined through long cycle testing. The system provided ≈ 6.8 kWh thermal energy output with a sorbent volume, Vs = 0.04 m3 (over a 1200 min discharge time), corresponding to an energy density, Ed = 170 kWh/m3. The required charging duration, to desorb the moisture was experimentally determined as 360 min. Based on the total energy input–output for both charging and discharging processes, the COPS was calculated at 2.39. According to the analysis, the experimental results were found in good agreement with the numerical simulation

Materials characterization of innovative composite materials for solar-driven thermochemical heat storage (THS) suitable for building application

Thermochemical Heat Storage (THS) systems have recently attracted a lot of attention in research and development. One of the main parameters that influence the performance of a THS system is the thermochemical materials. This paper aims to investigate thermochemical materials which are suitable for both short-term and long-term building heat storage application driven by solar energy for an open system. Innovative composite materials using MgCl2-MgSO4, CaCl2-LiCl and MgSO4-CaCl2salts mixtures impregnated into vermiculite, and potassium formate (KCOOH) impregnated into silica gel will be presented in this study. Initial screening and characterization results of the composite THS materials based on the energy density using differential scanning calorimetry analysis, mass loss against temperature using thermogravimetric analysis, and moisture vapor adsorption isotherms testing are discussed. The characterization analysis suggest that the vermiculite with salts mixtures are promising candidates for thermochemical heat storage (THS) systems compared to composite materials with individual salts. Meanwhile the potential of KCOOH-silica gel as THS materials may be further investigated in the future. The performance of the materials may be further optimized in the future by changing the concentration ratio of the mixed salts

Performance analysis and design implementation of a novel polymer hollow fiber liquid desiccant dehumidifier with aqueous potassium formate

A novel cross-flow liquid desiccant polymer hollow fiber dehumidifier (PHFD) is investigated numerically in this paper. The main objective of this research is to simulate, and validate the numerical model for future design implementations. The experimentally verified simulation data will be used to develop a set of design and implementation tables and charts as the guidance for selecting the number of fibres and the solution-to-air mass flow ratio of the PHDF under given conditions. A numerical model is developed to simulate the performance of the proposed innovative dehumidifier. This model is validated against three sets of data, i.e. the experimental obtained testing results, analytical correlations and the modelling results from the literature. The influence of various operating conditions such as inlet air properties (i.e. velocity, relative humidity) and inlet solution properties (i.e. temperature, concentration, mass flow rate) on the dehumidification sensible, latent, and total effectiveness, moisture removal rate are numerically analyzed. Dimensionless parameters including the number of heat transfer unit (NTU) and the number of mass transfer unit (NTUm), the solution to air mass flow rate ratio (m*), and the air to solution specific humidity ratio () have been used to evaluate the system performance. The results show that the increase in NTU and NTUm lead to a substantial change in dehumidification effectiveness. When the NTU increases from 0.47 to 7, the sensible effectiveness rises from 0.35 to 0.95. Increasing is another good option for increasing the amount of the absorbed moisture without influencing the latent effectiveness. For an increase of from 1.4 to 2.2, the air inlet and outlet specific humidity difference varies in the range of 0.008 kg/kg and 0.018 kg/kg