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.

In the 21st century the way to increase the efficiency of new sources of energy is directly related with extended exploration of renewable energy. This modern tendency ensures the fuel economy needs to be realized with nature protection. The increasing of new power sources efficiency (cogeneration, trigeneration systems, fuel cells, photovoltaic systems) can be performed by application of solid sorption heat pumps, regrigerators, heat and cold accumulators, heat transformers, natural gas and hydrogen storage systems and efficient heat exchangers.

An experimental investigation was performed on the thermal performance and heat transfer characteristics of acetone/zirconia nanofluid in a straight (rod) gravity-assisted heat pipe. The heat pipe was fabricated from copper with a diameter of 15 mm, evaporator-condenser length of 100 mm and adiabatic length of 50 mm. The zirconia-acetone nanofluid was prepared at 0.05–0.15% wt. Influence of heat flux applied to the evaporator, filling ratio, tilt angle and mass concentration of nanofluid on the heat transfer coefficient of heat pipe was investigated. Results showed that the use of nanofluid increases the heat transfer coefficient while decreasing the thermal resistance of the heat pipe. However, for the filling ratio and tilt angle values, the heat transfer coefficient initially increases with an increase in both. However, from a specific value, which was 0.65 for filling ratio and 60–65 deg for tilt angle, the heat transfer coefficient was suppressed. This was attributed to the limitation in the internal space of the heat pipe and also the accumulation of working fluid inside the bottom of the heat pipe due to the large tilt angle. Overall, zirconia-acetone showed a great potential to increase the thermal performance of the heat pipe.

A domestic hot water (DHW) system has been modernized in a multi-family house, located in the southeastern part of Poland, inhabited by 105 people. The existing heating system (2 gas boilers) was extended by a solar system consisting of 32 evacuated tube collectors with a heat pipe (the absorber area: 38.72 m2). On the basis of the system performance data, the ecological effect of the modernization, expressed in avoided CO2 emission, was estimated. The use of the solar thermal system allows CO2 emissions to be reduced up to 4.4 Mg annually. When analyzing the environmental effects of the application of the solar system, the production cycle of the most material-consuming components, namely: DHW storage tank and solar collectors, was taken into account. To further reduce CO2 emission, a photovoltaic installation (PV), supplying electric power to the pump-control system of the solar thermal system has been proposed. In the Matlab computing environment, based on the solar installation measurement data and the data of the total radiation intensity measurement, the area of photovoltaic panels and battery capacity has been optimized. It has been shown that the photovoltaic panel of approx. 1.8 m2 and 12 V battery capacity of approx. 21 Ah gives the greatest ecological effects in the form of the lowest CO2 emission. If a photovoltaic system was added it could reduce emissions by up to an additional 160 kg per year. The above calculations take also emissions resulting from the production of PV panels and batteries into account.

The present work involved an extensive outdoor performance testing program of a solar water heating system that consists of four evacuated tube solar collectors incorporating four wickless heat pipes integrated to a storage tank. Tests were conducted under the weather conditions of Baghdad, Iraq. The heat pipes were of 22 mm diameter, 1800 mm evaporator length and 200 mm condenser length. Three heat pipe working fluids were employed, ethanol, methanol, and acetone at an inventory of 50% by volume of the heat pipe evaporator sections. The system was tested outdoors with various load conditions. Results showed that the system performance was not sensitive to the type of heat pipe working fluid employed here. Improved overall efficiency of the solar system was obtained with hot water withdrawal (load conditions) by 14%. A theoretical analysis was formulated for the solar system performance using an energy balance based iterative electrical analogy formulation to compare the experimental temperature behavior and energy output with theoretical predictions. Good agreement of 8% was obtained between theoretical and experimental values.