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.     

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%.     

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.     

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.     

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.     

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.     

The impact of power of the auxiliary electric heater in the storage tank on the performance of a thermosyphon solar water system with an auxiliary electric heater (TSAEH)

When an auxiliary electric heater (AEH) is incorporated in a thermosyphon solar water system, the system performance becomes complex and difficult to simulate and analyse. Using the fixed-node, equally-convecting-mixing model, we have performed detailed simulations and analyses of the effects of the power of the auxiliary electric heater (PAEH) in the storage tank on the performance of a TSAEH and have also derived a formula for estimating the optimal value of the power of the auxiliary electric heater.     

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.     

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.     

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.     

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.     

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.     

Modelling of a thermosyphon solar water heating system and simple model validation

A thermosyphon solar water heater consisting of two flat plate collectors of total aperture area of 2.7 m² and 150 l storage tank is modelled using TRNSYS. Simple experiments were conducted in order to validate the model. During the experiments weather conditions were measured every 10 min and integrated over an hour. The temperature of the water in the storage tank was also measured at the beginning and at the end of the day. The storage tank temperature rise was used to validate the model by using the actual weather data as input to the program. Validation tests were performed for 25 days spread over 6 months and the mean deviation between the predicted and the actual values of water temperature rise is 4.7% which is very satisfactory. Subsequently, long term system performance is estimated by using TRNSYS model run with the weather values of TMY file for Nicosia, Cyprus. The annual solar fraction obtained was 79% and the system could cover all the hot water needs of a house of four people during the three summer months. The maximum auxiliary energy was needed during the months of December and January (about 280 MJ/month). In addition, an economic analysis of the system was carried out. The pay-back time of the system was found to be 8 years and the present worth of life cycle savings was found equal to C£ 161.     

Experimental investigation and simulation analysis of the thermal performance of a balcony wall integrated solar water heating unit

A balcony wall type solar water heater system was designed and constructed in a high-rise building. The U-type evacuated glass tube solar collector is fixed vertically on the balcony wall. The water, heated in the solar collector, flows through the exchanger coil in the water tank and then flows back to the solar collector. With regard to the hot water supply system, the cold water, heated by the heat exchanger, is sent to the point of use. Considering storeys and water consumption pattern, four apartments are selected for testing. Meanwhile, the theoretical analysis with TRNSYS was presented. According to the experimental results, mean daily collector efficiency is about 40%. Solar fraction is high in summer and autumn for the relative high radiation and high ambient temperature. Under given conditions, the annual energy extracted from tank is 2805.3 MJ/m², and the annual solar fraction is 40.5%. When the tank volume-to-collector area ratio is decreased to 37.5 L/m², the solar fraction can be increased to 50%. The results show that the family to use water all day round gets higher solar fraction than the family using hot water mostly in the morning and night.     

Experimental study of a thermosyphon solar water heater coupled to a fibre-reinforced plastic (FRP) storage tank

The thermal performance of the thermosyphon solar water heater was analyzed to show its applicability in a tropical climate using data of cloudy, sunny and hazy days. The average daily efficiency of the parallel-connected module, ranged between 35 and 40%. Also, an analysis of the temperature storage characteristics of a novel fibre-reinforced plastic (FRP) storage tank was undertaken. The inlet andoutlet positions were determined using the recommendation of Simon and Wenxian [1]: the optional position for the inlet/outlet was around the very top/bottom of the tank. The obtained results showed that the coupled FRP tank substantially retained and delivered the stored hot water during off-sunshine hours with minimal losses, and stratification occurred in the tank as a result. In view of the thermal performance, FRP materials can be efficiently employed in the design of solar hot water storage tanks.     

Economical and environmental assessment of an optimized solar cooling system for a medium-sized benchmark office building in Los Angeles,California

This paper presents a systematic energetic, economical, and environmental assessment on a solar cooling system for a medium-sized office building in Los Angeles, California by means of system modeling. The studied solar cooling system primarily consists of evacuated tube solar collectors, a hot water storage tank, a single-effect LiBr–H₂O absorption chiller, and a gas-fired auxiliary heater. System performance optimization and sensitivity analysis were conducted by varying two major parameters (i.e. storage tank volume and collector area). The results suggest that a trade-off exists between economic performance indicated by the equivalent uniform annual cost (EUAC) and the energetic/environmental performance indicated by the solar fraction and CO₂ reduction percentage, respectively. The cost of carbon footprint reduction was defined and served as an indicator for the overall system performance. Based on this indicator, the optimal system design could be found for a solar cooling system. The approach adapted in this study can be applied to other buildings located in different climate zones to reveal the cost and benefits of solar cooling technologies and facilitate decision-making.     

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.     

Performance of a solar-powered air conditioning system in Hong Kong

To study the feasibility of utilizing solar power for comfort cooling in Hong Kong, a solar-powered absorption air-conditioning system was designed and successfully constructed on the campus of the University of Hong Kong (HKU). The system consisted of a flat-plate collector array with a surface area of 38.2 m², a 4.7-kW nominal cooling capacity LiBr---H₂O absorption chiller, a 2.75-m³ hot-water storage tank, a cooling tower, a fan-coil unit, an electrical auxiliary heater, a data-acquisition system and the associated control systems. In the present paper, the design of the HKU solar-powered air-conditioning system is described in detail and its performance over an entire cooling season is also discussed and compared with similar systems in Italy and Singapore. It was found that the HKU solar air-conditioning system had an annual system efficiency of 7.8% and an average solar fraction of 55%.     

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.     

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.