Stanko Vl. Shtrakov, Anton Stoilov South — West University “Neofit Rilski”, Dept of Physics,

66 Ivan Mihailov Str., 2700 — Blagoevgrad, BULGARIA,

E-mail: sshtrakov@abv. bg, antonstoilov@abv. ba

Introduction — The major impediments for market penetration of solar hot water installations in Bulgaria are the lack of information and experienced data about the efficiency, thermal accumulation of energy and adequate exploitation in different seasons in a year and geographical regions. Defining useful recommendations for different regimes of exploitation, corresponding to the climatic conditions and installation parameters is the main purpose of this work. A special experimental solar module for hot water was built and equipped with sufficient measure apparatus. The main concept of investigation is to optimise the stratified regime of thermal accumulation and parameters of heat exchange equipment (heat serpentine in tank). Accumulation and heat exchange processes were investigated by theoretical end experimental means. Special mathematical model was composed to simulate the energy transfer in a stratified tank. Computer program was developed to solve mathematical equations for thermal accumulation and energy exchange. Experimental equipment with more than 15 temperature sensors and other measure devices gives data for the real processes. Extensive numerical and experimental tests were carried out. A good correspondence between theoretical and experimental data was arrived. Collected experimental and theoretical data from two years’ exploitation period is a good base for establishing some important issues about construction and exploitation of solar installation with different consumption regimes. Analysis of collected data were used to make a detailed technical and economical assessment of hot water installations with respect to the climatic and economical conditions in Bulgaria. This data will help designers and investors to expand penetration of the solar energy application in Bulgaria.

Solar hot water installations are the most often used solar applications in Bulgaria. The major impediments to further increase of the market penetration for these systems are the lack of information and experienced data about the efficiency, thermal accumulation of energy and adequate exploitation in different seasons in a year and geographical regions.

For small solar installations, used preliminarily in the domestic sector, the thermally stratified storage tanks for hot water is a good installation scheme. In such systems the hot water remains separated from the cold water by means of buoyancy forces. Stratified storage tanks are more thermally and economically effective. Maintaining thermal stratification is very important. This ensures that solar collectors work with maximal thermal efficiency. It is because the inflow to the collector is taken from the bottom of a stratified tank (the coldest layer in the tank). On the other hand, hot water for consummators is charged from the top of the accumulator, where the highest water temperature is kept. This delivers useful energy on demand.

A variety of models and experiments to assess the efficiency of stratified tanks have been produced. As a result many numerical and experimental studies have been conducted on the performance of stratified tanks under different operating conditions and constructive parameters. Most of the published studies analyze the direct solar installations or indirect installation with removed heat exchanger, where mass and thermal transport mechanism in accumulator and heat exchanger are separated. If an indirect solar installation with serpentine (included
heat exchanger) is used, the thermal exchange and accumulation perform simultaneously at the same place.

Solar installations with stratified tanks and heat exchange by included serpentine have advantages, because the destratification by fluid mixing in charge phase is eliminated. Only in discharge process the fluid mixing is available, but with some constructive measures the losses in stratification can be minimized. Moreover, the place of heat transfer in tanks can be regulated with serpentine disposition. The serpentine can be situated in upper, middle or bottom part of the tank. What is the right position of the heat exchanger (serpentine) is a disputable question. It depends on many factors and studies in this field will help designers and constructors of solar installations for hot water.

Motivated by this point, the present study is intended to investigate performance of typical domestic hot water installation in different regimes of thermal accumulation and climatic conditions. In order to wide scope for investments, a computational model and computer program for heat exchange process in accumulation tank was created. Many experimental and numerical studies have been conducted on the performance of stratified storage tank under different operating conditions and for different design characteristics. These characteristic include the parameters and situation on the heat exchange serpentine, flow rate in collector circuit, water consuming regime and other.