Long term performance of thermosyphon solar water heaters

The performance of 6 thermosyphon solar water heaters was measured while the systems were supplying typical domestic hot water loads. The effect of system configuration, daily load draw off times and off-peak vs continuous boosting was studied. In contrast to forced circulation systems the performance of thermosyphon systems was found to be best if a morning peak load pattern was used. Operation with off-peak boosting was found to improve the annual contribution by 14 per cent.     

Simulation of the long term performance of thermosyphon solar water heaters

A finite element simulation model for predicting the long term performance of thermosyphon solar water heaters is presented, and the simulation results are compared with the measured performance of six systems supplying typical domestic hot water loads. To obtain consistent simulation results the storage tank temperature stratification had to be accurately simulated using 20 nodes with 5 min time steps for vertical tanks and 30 nodes with 2 min time steps for horizontal tanks. A distributed return model for the mixing of the collector return flow in the tank was also found to be necessary to accurately model tank temperature stratification. The performance of single tank thermosyphon systems is shown to improve as the flow through the collector is reduced to approximately 1 tank volume per day and thermosyphon systems are shown to be slightly more efficient than equivalent pumped circulation systems.     

Performance rating method of thermosyphon solar water heaters

A rating method for the thermal performance of thermosyphon solar water heaters was developed. Except that the outdoor test procedure still follows the Taiwan Standard CNS B7277, a system characteristic efficiency η_s^* which is defined as the α₀ value corrected at , was derived so that η_s^* is independent of the ratio. Here, η_s^* can be evaluated by linear regression analysis of the test data. It is found from a series of tests for 31 systems that η_s^* is independent of indeed, and thus can be used to rate the thermal performance of different thermosyphon solar water heaters during the energy-collecting period. The cooling loss during the no-radiation period is rated by the system cooling time constant τ_c. The present rating method associated with the Taiwan Standard CNS B7277 has been implemented for more than three years and is accepted by the Taiwan solar industry.     

A criterion study of solar irradiation patterns for the performance testing of thermosyphon solar water heaters

A Taiwan test standard was established in 1989 using outdoor daily efficiency test methods. This test standard has been implemented for 12 years with satisfactory results. However, it was also found from field applications that the pattern of solar irradiation would affect the result of the performance test. In the present study, we used a distribution factor R_i defined as the ratio of the total irradiation in the morning to that in the afternoon to characterize this effect. R_i reflects the asymmetry of solar irradiation distribution in the morning and afternoon. A field study was carried out. The data collected from the daily efficiency tests were screened using the criterion of 0.5≤R_i≤1.6, in addition to the conditions defined in the Taiwan test standard. Two commercial products separately located in latitude 23° N and 25° N were tested. Data scattering occurs without using the R_i criterion. If we adopt test data using R_i, the results turn out to have a much better data correlation coefficient, from 0.915 to 0.969. The system characteristic efficiency η_s* changes significantly, from 0.479 to 0.514. There is a regulation that the commercial product should have a value of η_s* exceeding 0.5 in order to obtain a subsidy from the government in Taiwan. The performance test using the old standard is shown to result in a significant error, suggesting modification of the former test standard.     

An experimental investigation into the performance of a domestic thermosyphon solar water heater under varying operating conditions

An experimental investigation has been carried out on a thermosyphon solar water heater. The system consisted of a flat-plate collector of 1.5 m² absorber area with 21 tubes/m width and storage tank of 125 litre capacity. Experiments were carried out for both cloudy and clear weather conditions in winter and summer. The hourly system performance was evaluated for all test conditions. The final mean tank temperature was measured daily which enabled the calculation of the possible contribution of solar energy for domestic hot water supply in Basrah, Iraq (latitude 30.76°N). The system was tested at both no-load and loading conditions. Intermittent and continuous load was imposed, and system performance was evaluated for each condition.     

System modeling and operation characteristics of thermosyphon solar water heaters

An efficient numerical simulation model for thermosyphon solar water heaters has been developed and compared with test data from two locations. The model was used to study the characteristics of vertical and horizontal tank thermosyphon systems. The results indicate that thermosyphon systems have optimum performance when the daily collector volume flow is approximately equal to the daily load volume. Heat conduction in one tank horizontal system was found to significantly reduce solar contribution.     

Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

Aqueous propylene-glycol concentrations for the freeze protection of thermosyphon solar energy water heaters

Using a validated dynamic simulation model, the thermal performance of an indirect thermosyphon solar energy water heater was examined. The heat transfer fluids employed were aqueous solutions of propylene glycol. The effect of varying the glycol concentration on the hot water output and efficacy of freeze protection was determined for a specific pattern of hot water withdrawal and weather for the temperate maritime climate of London, England. The heat output is compared with that of a drain-down direct system.     

Design and fabrication of low cost solar water heaters

We designed two types of very low cost solar water heaters which do not need a water supply connection. The first one consisted of two plastic bowls, one inside the other with 5 cm thickness of insulation in between. A transparent plastic cover was tied around the smaller vessel. It was found that water placed inside the smaller vessel had its temperature raised by 18°C from the ambient temperature within four hours when the average insolation was about 600 kWh/m². The second one consisted of two earthen vessels in place of plastic bowls. The water temperature rise was 20°C under the same radiation. If either of the heaters was covered with cotton-wool insulation in the afternoon, the temperature remains 13°C above the ambient early next morning. The cost of the material was around US$ 4.50 for the plastic bowl heater while this is around US$ 3.50 for the earthen vessel water heater.     

Experimental investigation of a two-phase closed thermosyphon solar water heater

In this study, experiments were performed to find out how the thermal performance of a two-phase thermosyphon solar collector was affected by using different refrigerants. Three identical small-scale solar water heating systems, using refrigerants R-134a, R407C, and R410A, were constructed and tested side-by-side under various environmental and load conditions. The performance of the system under clear-sky conditions has been investigated with and without water load. Detailed temperature distributions and cumulative collection efficiencies were determined and presented. The experimental results were compared to the results found in the literature and they showed good agreement.     

Long-term thermal performance of a two-phase thermosyphon solar water heater

This article investigates experimentally the long-term thermal performance of a two-phase thermosyphon solar water heater and compares the results with the conventional systems. Experimental investigations are conducted to obtain the system thermal efficiencies from the hourly, daily and long-term performance tests. Different heat transfer mechanisms, including natural convection, geyser boiling, nucleate boiling and film-wise condensation, are observed in the two-phase thermosyphon solar water heater while solar radiation varies. The thermal performance of the proposed system is compared with that of four conventional solar water heaters. Results show that the proposed system achieves system characteristic efficiency 18% higher than that of the conventional systems by reducing heat loss for the two-phase thermosyphon solar water heater.     

Thermal performance of plastic film solar air and water heaters

The results of measurements on a few solar air and water heaters of the same design, but made from different plastic materials, are reported for different climatic conditions. The experiments on the systems were made over a long time period, first in Germany in 1982 and then in India in 1984. An economic evaluation shows that the use of such collectors can bring down the cost of solar energy in the desired economic scale. Despite the drawback of UV degradation of plastic material resulting in shorter lifetime, the low initial investment cost keeps the cost of useful energy also low.     

Year round performance of thermosyphon solar water heater in Benghazi

The relative merits of a thermosyphon heater for domestic water heating are brought to light with special emphasis to Libyan weather conditions. The experimental performance of this type of heater was studied by the authors for a brief period and reported in an earlier study. In the present study, year-round experimental performance is reported. Measurements performed all over the year showed that water is heated to a maximum temperature of 66°C while the minimum temperature during six months was as high as 50°C. The picture is no less bright from an overall efficiency point of view. Efficiency ranges as high as 40 to 50%. Hourly measurements on various days showed that the water temperature has an increasing trend, irrespective of the fluctuating solar radiation pattern on a cloudy day.     

Analytical approach of thermosyphon solar domestic hot water system performance

An efficient and simple simulation approach for thermosyphon solar water heaters has been developed and compared with experimental data. This approach, valid for solar-only systems, gives the ability to link the system main design and constructional parameters with the expected energy output through an analytical determination of the coefficients of the characteristic input–output equation of the system. The proposed methodology can be used not only for energy optimization of the system in the design phase but also for evaluation of test results of an existing system in order to improve it further.     

Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

Comparative study of three thermosyphon solar water heaters made of flat-plate collectors with different absorber configurations

The paper presents a comparative study of three solar water heaters made of flat-plate collectors with different absorber configurations. The performance of the three solar water heaters is assessed under the same conditions. The collectors have the same surface area and are glazed. The theoretical model for each collector type, with the transient effects taken into account, is based on a control volume. By considering a small element of the collector in each case and the storage tank, six partial differential equations were developed for each solar water heater and were solved numerically for a cloudy day. This study shows that the thermal performance obtained with the solar water heater using the absorber-pipe lower bond configuration in the solar collector is always greater than the two others. These results showed that the solar water heater made of the absorber-pipe lower bond configuration is more efficient than the other systems.     

Comparison of experimental and calculated performance of integral collector-storage solar water heaters

Experimental measurements of the monthly performance of an integral collector-storage solar water heater for a one-year period are compared with performance predictions using the method of Zollner. The prediction method requires two parameters which were obtained from indoor experiments with a solar simulator. The experimental measurements are also compared with predictions in which the two parameters were obtained from short-term outdoor tests.     

Transient investigation of a two phase closed thermosyphon flat plate solar water heater

This work presents an unsteady state theoretical and experimental investigation of natural circulation two phase closed thermosyphon flat plate solar water heaters. The governing equations of the heater storage tank and connecting pipes are presented and generalized in dimensionless form, while the governing equations of the different components of the collector were previously discussed by the author. Also, the author's earlier simulation program of the collector is modified to include the solution of the dimensionless governing equations of the present analysis. For verifying the modified simulation program, a two phase closed thermosyphon solar water heater is designed, constructed and tested at different meteorological conditions, initial storage tank temperatures and different hot water withdrawal load patterns. The comparison between the experimental results and their corresponding simulated ones shows considerable agreement.     

The theoretical and experimental investigation of the phase-change solar thermosyphon

The paper is concerned with the study of two-phase solar thermosyphon for domestic hot water (SDHW). Due to a combination of ideas of the heat pipe and classical thermosyphon, the heat transfer in the proposed installation is to be realised by the liquid - gas phase change, what should increase significantly both the heat efficiency of the system and the system applicability.