• Energy Research

Abstract The phase change of sodium acetate (SA) aqueous solution to sodium acetate trihydrate (SAT) requires large supercooling degree, then the aqueous solution can be at liquid state at fairly low temperature without releasing the stored latent heat. Such a feature makes SAT a promising material for seasonal solar thermal energy storage. The present study firstly summarized the thermo-physical properties of the solid SAT and liquid SA aqueous solution at different temperatures and concentrations, including equilibrium temperatures, densities, specific heats and thermal conductivities. The calculation methods of these properties have been established. Secondly, with the aid of the above properties, a mathematic model of the thermal discharge process of the storage system, i.e. the solidification process of supercooled SA aqueous solution, was built based on the heat transfer between the phase changing material within a single storage tube and the external flowing heat transfer fluid (HTF). The experimentally obtained SAT crystal growth rate and the enthalpy change of solidifying supercooled SA aqueous solution were employed to aid the modelling. The discharge temperature and thermal power of the storage system were numerically obtained and analysed. The influence of the ambient temperature, the mass flow rate as well as the heat transfer coefficient of the HTF on the thermal discharge performance were discussed. Finally, the seasonal thermal storage density of SAT was given and compared to that of water and some sorption materials.

Abstract Seasonal solar thermal energy storage (SSTES) has been investigated widely to solve the mismatch between majority solar thermal energy in summer and majority heating demand in winter. To study the feasibility of SSTES in domestic dwellings in the UK, eight representative cities including Edinburgh, Newcastle, Belfast, Manchester, Birmingham, Cardiff, London and Plymouth have been selected in the present paper to study and compare the useful solar heat available on dwelling roofs and the heating demand of the dwellings. The heating demands of space and hot water in domestic dwellings with a range of overall heat loss coefficients (50 W/K, 150 W/K and 250 W/K) in different cities were calculated; then the useful heat obtained by the heat transfer fluid (HTF) flowing through tilted flat-plate solar collectors installed on the dwelling roof was calculated with varied HTF inlet temperature (30 °C, 40 °C and 50 °C). By comparing the available useful heat and heating demands, the critical solar collector area and storage capacity to meet 100% solar fraction have been obtained and discussed; the corresponding critical storage volume sizes using different storage technologies, including sensible heat water storage, latent heat storage and various thermochemical sorption cycles using different storage materials were estimated.

Abstract Hydration is an alternative promising method for CO2 capture and separation from either post-combustion flue gas or pre-combustion fuel gas. The present paper gathers the researches on CO2 hydrate and the hydrates of gas mixtures of CO2+N2/H2/CH4, including studies of fundamental thermo-physical properties, molecular structures and hydrate formation equilibrium conditions. Some promoters, i.e. quaternary ammonium salt etc. are usually used in CO2 hydration process to reduce the hydrate equilibrium pressure and to enhance the hydrate kinetic and stability, hence their promotion effect on CO2 hydrate and on the hydrates of gas mixture of CO2+N2/H2/CH4 are reviewed. The paper also summarizes the applications of hydrate technology in CO2 capture and separation, and the corresponding performance is summarized and the bottlenecks are discussed. It necessitates more works to promote this technology towards industrial application.

This paper presents an optimised resorption cogeneration using mass and heat recovery to improve the performance of a novel resorption cogeneration fist proposed by Wang et al. This system combines ammonia-resorption technology and expansion machine into one loop, which is able to generate refrigeration and electricity from low-grade heat sources such as solar energy and industrial waste heat. Two sets of resorption cycle are designed to overcome the intermittent performance of the chemisorption and produce continuous/simultaneous refrigeration and electricity. In this paper, twelve resorption working pairs of salt complex candidates are analysed by the first law analysis using Engineering Equation Solver (EES). The optimal resorption working pairs from the twelve candidates under the driven temperature from 100 °C to 300 °C are identified. By applying heat/mass recovery, the coefficient of performance (COP) improvement is increased by 38% when the high temperature salt (HTS) is NiCl2 and by 35% when the HTS is MnCl2. On the other hand, the energy efficiency of electricity has also been improved from 8% to 12% with the help of heat/mass recovery. The second law analysis has also been applied to investigate the exergy utilisation and identify the key components/processes. The highest second law efficiency is achieved as high as 41% by the resorption working pair BaCl2–MnCl2 under the heat source temperature at 110 °C.

This paper presents a thermodynamic analysis and a numerical simulation of a heat pipe heat exchanger which recovers both sensible and latent heat from the exhaust gases of boiler with a temperature range from 450K to 600K. Compared with the conventional methods of preventing corrosion by avoiding acid dew point or using the anticorrosive material, a water spray is proposed in this work as an innovation to integrate with the heat pipe heat exchanger, which absorbs the corrosive gas such as SO2, SO3 and NOx from the outlet of boiler. The comprehensive theoretical study has shown the convective heat transfer coefficient under wet condition is 1.5-3 times higher than that of dry condition and the optimal location of the water spray in the system has been identified. Meanwhile a the heat and mass transfer in a thirty-row heat pipe heat exchanger with different locations of a water spray has been established by the FLUENT to analyze the flow field and temperature gradient of the heat pipe heat exchanger. The overall analysis has proven that system efficiency of the boiler and the lifetime of heat exchanger can be effectively enhanced with the application of the water spray.

Abstract To maximise the utilisation of solar energy and improve the solar fraction for domestic applications, this paper explored the potential of the hybrid solar Photovoltaic/Thermal (PV/T) collector integrated with a thermochemical sorption thermal storage system. The thermal output was used to provide domestic hot water or stored over seasons in the England city of Newcastle upon Tyne. The performance of the water-cooled PV/T collectors with or without an air insulation layer between the glass cover and the Photovoltaic (PV) cell was compared. The electrical power generation model of the PV cell developed in MATLAB was coupled with a Computational Fluid Dynamics (CFD) model to simulate the simultaneous generation of electrical and thermal energy. The one-diode model was used to simulate the electrical production of the PV cell with the new correlations of the series resistance and the shunt resistance proposed in this work, so that the accuracy of dynamic performance simulation can be improved especially in the cases with relatively higher PV cell temperature. The water outlet temperature was studied at 100 °C to meet the heat supply requirement of the sorption cycle using the working pair strontium chloride-ammonia. It was found that the PV/T collector with air gap could produce 28 L hot water per day per m2 collector (L/(day·m2)) with the electric efficiency of about 10% if the water outlet temperature was required at 100 °C; in contrast, around 133 L/(day·m2) was produced with the electric efficiency of 13% when the water outlet temperature at 40 °C. The PV/T collector without air gap was not competent for the applications studied in this work especially in cold regions. The application case studies suggested that an installation of 26 m2 air-gap PV/T collectors integrated with the strontium chloride-ammonia thermochemical sorption storage system can fully satisfy the annual hot water demand of an ordinary single household in Newcastle upon Tyne with 100% solar sources, and cover at least half of the annual electricity consumption.

AbstractOver the last decade, energy prices in China have risen dramatically. At the same time, extensive use of coal fired energy provision systems in industry has led to serious environmental and economic problems translating to an economic damage of an estimated 10% of the Gross Domestic Product. This has led to increasing awareness in the process industries of the need to save energy whilst replacing conventional energy sources with renewable ones.An energy audit was conducted for a soy sauce production facility in Beijing, which aimed to reduce its thermal energy demand through process intensification and to integrate renewable energy. Their current supply of thermal energy came directly from a district steam network, which was both directly consumed and downgraded via heat exchangers. It was determined that the best two solar integration locations would be in the pre-heating/mixing of raw ingredients to 60°C and the subsequent direct steaming of the mixture to 120°C.Three different systems for supplementing steam were investigated: (1) a traditional solar thermal heating system; (2) a system consisting of mono crystalline photovoltaic panels coupled with either a resistance heater or electric steam generator; and (3) a cascading system consisting of two types of solar thermal collectors, photovoltaic panels, and an electric steam generator. Comparisons of systems 1 and 2 were made for the heating of mixing water, and systems 1, 2, and 3 for saturated steam generation.Results showed that for the heating of process water, flat plate solar collectors performed best with an estimated 20year Levelised Cost of Energy of 0.063€/kWh. Steam generation was most cost effective with a cascade system of photovoltaic and flat plate collectors, with an estimated 20year Levelised Cost of Energy of 0.145€/kWh. The model predicts that integration of this technology would lead to a reduction of 14% in heating utility demand.