Effects of auxiliary heater on annual performance of thermosyphon solar water heater simulated under variable operating conditions

A thermosyphon solar water heating system with electric auxiliary heater was simulated using the TRNSYS simulation program. Location of the auxiliary heater, inside the storage tank or connected in series between the system and the user, was studied using the TMY meteorological data for Los Angeles, California. Simulations were performed for two different water load temperatures (60 and 80°C) and for two types of daily hot water volumes (250 and 150 l). Four types of daily hot water consumption profiles were used in the present study, namely; the widely used Rand profile, continuous, evening and morning profiles. Also, the simulation is extended to cover the effects of thermal and optical properties of the flatplate collector and the volume of the storage tank. The results show that if water is drawn on a schedule corresponding to the Rand draw profile, the system operates with higher efficiency when the auxiliary heater is located in the storage tank than when the auxiliary heater is outside the storage tank. When operated with each of the other three draw schedules, however, better performance is achieved by locating the auxiliary heater outside the tank. The increase in solar fraction depends on the load profile and volume, temperature setting, as well as the quality of the collector and the storage tank volume. When the values of the parameters F_R(τα)_n and F_RU_L are changed from 0.8 and 16 kJ/h m²°C to 0.6 and 30 kJ/h m²°C, the solar fraction decreases by approximately 40–50%.     

Simulation studies of the position of the auxiliary heater in thermosyphon solar water heating systems

This paper investigates the effect of the physical location of the auxiliary source of energy in thermosyphon solar water heaters and shows that the performance of the system can be optimised with respect to the geometry of the system components. The investigation has been based on a domestic thermosyphon solar water heating system, which was simulated using the TRNSYS programme. The annual solar fraction of the system, at the weather and socioeconomic conditions of Cyprus, is, at best, approximately 77% with an in-tank auxiliary heater configuration and 86% with an external auxiliary heater. It is demonstrated that the arrangement with the external auxiliary unit has a higher collector efficiency and results in a higher annual solar fraction. In the case of in-tank auxiliary, the system performance increases with the height of the auxiliary position from the bottom of the storage tank; with the auxiliary at the bottom of the storage tank the annual solar fraction is approximately 59%, compared to 77% when the auxiliary is located at the top of the tank. The system performance also depends on the height of the collector return from the bottom of the tank.     

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.     

Effects of stratification on the performance of liquid-based solar heating systems

A theoretical model is presented which predicts the effects of storage tank stratification on the instantaneous performance of a liquid-based solar heating system. The results are presented in terms of a stratification coefficient which is defined to be the ratio of the actual useful energy gain to the energy gain that would be achieved in a fully mixed tank. This stratification coefficient is shown to be a system constant which depends on only two dimensionless system parameters. The closed form model is compared with a detailed numerical simulation and also with experimental data taken with a solar water heater. Both the simulation data and the experimental data agree favorably with the theoretical model.     

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.     

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.     

Analysing the effect of the laeh in the storage tank on the performance of a tsaeh

When an auxiliary electric heater (AEH) is incorporated in a thermosyphon solar water system, the system performance becomes more complex and it is more difficult to simulate and analyse its operational characteristics. Using a ‘fixed-node, equally-convecting-mixing model’ that they have developed, the authors perform detailed simulations and analysis of the effects of the location of the auxiliary electric heater (LAEH) in the storage tank on the performance of the thermosyphon solar water system with an auxiliary electric heater (TSAEH), and discover for the first time some new laws governing the system operation. The authors believe that their simulations and analysis will be very beneficial for optimal design and practical application of such systems in the future.     

OPTIMIZATION OF DOMESTIC SOLAR WATER HEATING SYSTEM

TRNSYS program was used to simulate the performance of a thermosyphon type domestic solar water heating system which is used in the West Bank. The system installation and operation parameters were optimized including collector tilt angle, the hot water storage tank volume and location relative to the collector, and the hot water consumption pattern and daily rate.     

The optimization of tank-volume-to-collector-area ratio for a thermosyphon solar water heater

Through the use of the TRNSYS simulation program, the performance of a domestic solar water heating system operating with natural circulation (thermosyphon) and a daily hot water load has been analysed. The effect of tank height on the annual solar fraction of the system has been investigated for different hot water load temperatures and storage tank volumes. Optimum values (values which maximize the annual solar fraction of the system) for storage tank height and volume are calculated for operating temperatures ranging from 50 to 80°C. The response of the system to the ratio of the storage tank volume to the collector area is investigated. The dependence of the solar fraction on tank height was observed to be more notable in the case of large tank volumes and high load temperatures. The results indicate the existence of an optimum value for the tank volume at a given tank height and a high load temperature. At lower temperatures, the solar fraction rises rapidly with tank volume to a nearly constant level. An optimum value of the storage-tank-volume-to-collector-area ratio was also observed at high load temperatures.     

Performance characteristics of integral type solar-assisted heat pump

The characteristic of an integral type solar-assisted heat pump water heater (ISAHP) is investigated in the present study. The ISAHP consists of a Rankine refrigeration cycle and a thermosyphon loop that are integrated together to form a package heater. Both solar and ambient air energies are absorbed at the collector/evaporator and pumped to the storage tank via a Rankine refrigeration cycle and a thermosyphon heat exchanger. The condenser releases condensing heat of the refrigerant to the water side of the thermosyphon heat exchanger for producing a natural-circulation flow in the thermosyphon loop. A 105-liter ISAHP using a bare collector and a small R134a reciprocating-type compressor with rated input power 250 W was built and tested in the present study. The ISAHP was designed to operate at an evaporating temperature lower than the ambient temperature and a matched condition (near saturated vapor compression cycle and compressor exhaust temperature <100°C). A performance model is derived and found to be able to fit the experimental data very well for the ISAHP. The COP for the ISAHP built in the present study lies in the range 2.5–3.7 at water temperature between 61 and 25°C.     

Technical note Best connection scheme of collector modules of thermosyphon solar water heater operated at high temperatures

Large scale thermosyphon solar water heater for high temperature applications is simulated by the use of the Transient Simulation Program (TRNSYS). A daily hot water load of 1500 l/day and 2500 l/day at 80°C was assumed. The hot water is consumed daily from 08·00–17·00 h. A back-up electric auxiliary heater was added to the system in two schemes: first, located inside the storage tank with a thermostat; second, outside the tank connected to the heating system between the tank and the facilities. The collector modules were connected in five different schemes: first, all collectors were connected in series in one line, or collectors were connected in two, three, four or five parallel lines each consisting of many collectors. The results showed that the best connection is when the 20 collectors, comprising the system, are connected in two parallel lines each consisting of 10 collectors. It was found that the monthly and yearly useful energy from the system was higher when the auxiliary water heater was added to the system outside the storage tank.     

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.     

Simulation of a solar domestic water heating system using a time marching model

This paper presents the modelling and simulation of a solar water heating system using a time marching model. The results of simulations performed on an annual basis for a solar system, constructed and operated in Yugoslavia, which provides domestic hot water for a four-person family are presented. The solar water heater consists of a flat-plate solar collector, a water-storage tank, an electric heater, and a water-mixing device. The mathematical model is used to evaluate the annual variation of the solar fraction with respect to the volume of the storage tank, demand hot water temperature required, difference of this temperature and preset storage tank water temperature, and consumption profile of the domestic hot water demand. The results of this investigation may be used to design a solar collector system, and to operate already designed systems, effectively. The results for a number of designs with different storage tank volumes indicate that the systems with greater volume yield higher solar fraction values. The results additionally indicate that the solar fraction of the system increases with lower hot water demand temperature and higher differences between the mean storage water and the demand temperatures. However, when a larger storage tank volume is used, the solar fraction is less sensitive to a variation of these operation parameters.     

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.     

Performance analysis of solar water heater combined with heat pump using refrigerant mixture

In the research presented in this paper the thermal performance of a solar water heater combined with a heat pump is studied. A solar collector was modified from corrugated metal roofing with a copper tube attached beneath. The performance of the solar water heater was tested, and models for the collector efficiency and storage tank were developed and used for the evaluation of their performance when combined with a heat pump system.     

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.     

带透明蜂窝盖板和辅助反射面的整体式太阳热水器

作者对一种带透明蜂窝盖板和辅助反射面的整体式(ICS)太阳热水器进行了实验研究。该太阳热水器采用截面为三角形的水箱，水箱背面和侧面用30mm聚苯乙烯泡沫隔热，其它两个面为吸热面。底吸热面利用辅助反射面加热，而上吸热面则覆盖5cm的透明蜂窝及2mm的有机玻璃板。这种设计加大了ICS太阳热水器的吸热面积，同时也降低了吸热面向环境的热损。对实验结果的分析表明，该热水器的热效率不高，但保温性能很好。     

Modeling of the CSU heating/cooling system

This paper resents a thermal simulation of the Colorado State University solar house. A computer model of the solar energy system was developed and computer runs were made using one year of meteorological data to determine the important design features. The system consists of a flat plate solar collector, main storage tank, service hot water storage tank, auxiliary heater, absorption air conditioner with cooling tower and heat exchangers between the collector and storage, storage and service hot water tank and storage and residence. This system very closely models the CSU house in operating mode one.The results are in the form of monthly integrated values for the pertinent energy quantities. In addition, results are presented which show the effect on the system performance of the collector tilt, collector area and number of covers.     

Study of a solar water heater using stationary V-trough collector

There are various types of solar water heater system available in the commercial market to fulfill different customers’ demand, such as flat plate collector, concentrating collector, evacuated tube collector and integrated collector storage. A cost effective cum easy fabricated V-trough solar water heater system using forced circulation system is proposed. Integrating the solar absorber with the easily fabricated V-trough reflector can improve the performance of solar water heater system. In this paper, optical analysis, experimental study and cost analysis of the stationary V-trough solar water heater system are presented in details. The experimental result has shown very promising results in both optical efficiency of V-trough reflector and the overall thermal performance of the solar water heater.     

Experimental and analytical performance study of a thermosyphon water heater

The performance of a thermosyphon solar water heater was studied analytically and experimentally. A finite-difference model was used to predict year-round performance. Tests were conducted on an experimental heater subjected to acutal weather conditions in Benghazi, Libya. Satisfactory qualitative and quantitative agreement was found between experimental and predicted results. A storage volume of 60 liters per unit collector area was found to be optimum for Benghazi conditions. The day-end temperature was found to vary between 23° and 51°C for the test period, which occurred in winter, with an average of 41°C.