The article presents a numerical model of the concrete heat accumulator for solar heating systems. Model uses control volume finite element method with an explicit solution method for time integration. The use of an explicit method is an essential advantage in the simulation of time-dependent changes in temperature of the air at the accumulator inlet. The study compares the results of numerical model calculations of the accumulator heating with experimental measurements and with computational fluid dynamics modeling. The comparison shows a good correlation between the results of calculation using the model and the results of measurements.

Solar collectors are used increasingly in single-family housing. Their popularity depends on many factors, including the price-to-productivity ratio, which in turn results from the development of solar collector technology as well as entire systems. This development consists of many aspects, including those related to the modernization of control systems and measuring of solar collector systems. Currently used systems offer, among others, the ability to determine the approximate solar heat gains using the sensors necessary for normal control of the sensor system. The paper analyzes, on the example of one facility, how such installations work in Polish conditions. An installation consisting of 3 solar collectors has been selected for analysis, supporting the preparation of hot utility water for a single-family residential building. The detailed analysis concerned days with high heat gains compared to the average heat demand for hot water preparation in the building. The temperature verification method (TVM) of the calculated solar heat gains by the solar system controller has been proposed. Then, differences in measurements according to two methods (controller and TVM) have been presented at various characteristic moments of the installation’s operation (start- -up, stop) and during continuous operation. It has been shown that during the day gains measured by the controller can be 15% lower than gains measured by the TVM method. The check has been carried out at a daily sunlight value higher than 4.8 kWh/m2 measured on a horizontal plane. The ratio of heat energy supplied to the domestic hot water storage tank to the measured insolation has been 34%. The sum of annual solar heat gains measured by the controller and TVM differed by 5.2%.

The performance of ten wickless heat pipes without adiabatic sections is investigated experimentally at low heat inputs 120 to 2000 W/m2 for use in solar water heaters. Three heat pipe diameter groups were tested, namely 16, 22, and 28.5 mm. Each group had evaporator lengths of 1150, 1300, and 1550 mm, respectively, with an extra evaporator length of 1800 mm added to the second group. The condenser section length of all heat pipes was 200 mm. Ethanol, methanol, and acetone were utilized as working fluids, at inventory of 25%, 50%, 70%, and 90% by evaporator volume respectively. The 22 mm diameter pipes were tested at inclination angles 30◦, 45◦, and 60◦. Other diameter groups were tested at 45◦ only. Experiments revealed increased surface temperatures and heat transfer coefficients with increased pipe diameter and evaporator length, and that increased working fluid inventory caused pronounced reduction in evaporator surface temperature accompanied by improved heat transfer coefficient to reach maximum values at 50% inventory for the selected fluids. Violent noisy shocks were observed with 70% and 90% inventories with the tested heat pipes and the selected working fluids with heat flux inputs from 320–1900 W/m2. These shocks significantly affected the heat pipes heat transfer capability and operation stability. Experiments revealed a 45◦ and 50% optimum inclination angle of fill charge ratio respectively, and that wickless heat pipes can be satisfactorily used in solar applications. The effect of evaporator length and heat pipe diameter on the performance was included in data correlations.