Over the past several decades, much attention has been given to the development of technologies utilizing solar energy to generate inexpensive and clean heat for heating purposes of buildings and even for electricity generation in the concentrating solar thermal power (CSTP) plants. However, unlike conventional heatgenerating technologies consuming coal, natural gas, and oil, heat produced by solar energy is intermittent because it is significantly affected by daily (day-night) and seasonal fluctuations in solar insolation. This fact issues a considerable challenge to the adoption of solar energy as one of the main renewable heat sources in the future. Therefore, along with the development of the different solar technologies, the heat storage technologies have also been the focus of attention. Use of the storage devices, able to accumulate heat, enables not only enhance the performance of the heating systems based on solar energy but also make them more reliable. This paper gives an overview of the various sensible heat storage technologies used in tandem with the fluctuating solar heat sources.

The paper presents the results of an experimental study of the exothermal and endothermal effects occurring on the dissolution of potassium nitrate KNO3, ammonium nitrate NH4NO3, potassium hydroxide KOH and sodium hydroxide NaOH, in distilled water with natural mixing and forced stirring. It is shown that for natural mixing, NH4NO3 and KOH are the most appropriate working substances for cooling and heat generation respectively, while for forced stirring, NH4NO3 and NaOH display better performance. The optimal concentrations of the liquid solutions for achieving extreme temperatures and maximal enthalpy changes are also defined. In addition, the regression function and regression coefficients describing correlations between the total enthalpy change of the calorimetric system and the solution concentration are found, for the most suitable liquid solutions. The experimental results obtained can be used to develop generating reactors for exothermal heating and endothermal cooling for a thermochemical seasonal storage system able to store solar heat or heat from combined heat and power (CHP) units of biogas plants in summer, for heating purposes in winter, while generating cooling to support air conditioning systems in summer.

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The paper presents the results of an experimental study on the exothermal heat and endothermal cooling generation under vacuum conditions using a water distillation system of special design. The heat and cooling are generated when KOH and NH4NO3 crystals are dissolved as a result of natural mixing in the water, distilled from NH4NO3 and KOH liquid solutions, respectively. The time evolutions of the temperatures measured in KOH→H2O→NH4NO3 and NH4NO3→H2O→KOH experiments are presented and discussed. It has been shown that under vacuum conditions, the endothermal effect of NH4NO3 dissolution in water is stronger than that observed at atmospheric pressure, while the exothermal effect of KOH dissolution in water is weaker. The experimental results obtained will be used to develop the dual thermochemical heat storage and cooling generating system in order to utilize solar heat in the summer.