Thermal optimisation of a built-in storage solar water heater

A simple transient analysis of a built-in storage solar water heater is presented; the results obtained by the present theory are in close agreement with experimental observations as well as with predictions obtained by a more rigorous theory. The water heater consits of an insulated rectangular tank whose top surface is suitably blackened by blackboard paint and then glazed. The unit is exposed to solar radiation during sunshine hours and is covered by adequate insulation at night. The effect of insulation thicknesses (top and bottom insulation) and water mass in the tank is discussed. The importance of using a reflecting sheet instead of insulation is emphasised.     

Performance of a solar collector/storage water heater

This paper presents a general analytical model of a solar collector/storage water heater in which water is flowing at a constant rate between the glass cover and the absorbing plate. The effects of the variations of the depth of water, its flow velocity and the length of the absorbing plate on the performance of the collector have been studied.     

Effect of heat exchanger on the performance of a shallow solar pond water heater

In this communication, a mathematical model has been developed to predict the performance of a shallow solar pond water heater with a heat exchanger. Explicit heat balance equations are written for the plate temperature and water tank temperature, as well as for the heat extracting fluid temperature, by properly taking into account the absorption of solar radiation in the body of pond water. It is seen that efficiencies may be achieved as high as 60% at water flow rates of 0.1 – 0.2 kg/s m². Thereafter, the efficiency becomes almost constant at higher flow rates.     

Comments on ‘a simplified theory for a matrix solar collector’

A critical analysis is given of Singh and Bansal's paper (1983) ‘A simplified theory for a matrix solar collector'. The following points are demonstrated:

• 1 Singh and Bansal use the equation of heat conduction to model an experimental system in which transfer of heat by conduction plays very little part. They make no attempt to show that the true radiative transfer equations can be approximated by a conduction equation.
• 2 The units and dimensions of the equations are internally self-inconsistent.
• 3 Correcting the equations to make them dimensionally consistent, and using the data given by Singh and Bansal, their calculations cannot be replicated. The values for outlet temperature are in the region 4°C rather than the 12°C claimed by Singh and Bansal.
• 4 Some of the data values quoted by Singh and Bansal are wrong (in terms of their stated objective of using values corresponding to the experimental system of Chau and Henderson) and others appear to have been assigned in an extremely arbitrary fashion. One crucial parameter (the absorptivity of the matrix material) is not even mentioned in Singh and Bansal's analysis.
• 5 Using the correct values for those parameters referring to the experimental system, and including the effect of absorptivity, the theory can be made to give results of the same order of magnitude as the experimental results. However, the variation of the outlet temperature as a function of the experimental parameters is very different to that obtained from Chau and Henderson's computer simulations. Indeed, Singh and Bansal appear not to have noticed that their solution possesses a singularity, so that for certain values of the parameters, an infinite outlet temperature can be obtained. This would appear to invalidate their model as a serious analysis of the matrix air heater.

     

The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer

We present a theoretical study of a solar photovoltaic-thermal (hybrid) system consisting of a flat-plate solar air heater mounted with solar cells and a plane booster. A conventional flat-plate collector is converted into a hybrid system by mounting solar cells directly on the absorber plate. A hybrid system is self-sufficient in the sense that the electrical energy required by the pump is supplied by the panel. Such systems are well suited to applications such as solar drying. The combined system is analysed for the case when the radiative and absorptive properties of the cell surface and the absorber plate are nearly the same. The solar cell efficiency is a linearly-decreasing function of the absorber plate temperature. The performance of the system has been evaluated for various combinations of boosters. The minimum area of the solar cells required to run the pump at a given flow rate has been calculated as a function of time, with and without boosters. The minimum cell area required decreases with the use of boosters. High cost cells may be replaced by low cost reflectors. The solar air heaters presently available on the market are not suitable for direct conversion to hybrid systems.     

Design and performance of a large-size solar water heater

The paper reports the design details of a solar water heater suitable for the large, intermittent demands for hot water by hospitals and hostels. It employs a flat-plate collector consisting of a wire-tied aluminium fin of 28 gauge with galvanized iron pipes of 19 mm diam spaced at 10 cm centres. The unit is adjusted to give maximum efficiency per unit cost under Indian conditions. Various arrangements for connecting the collectors, such as cascade, series, series parallel, and true parallel, were experimentally studied. This revealed that for a solar water heating system having a large number of absorber banks, the true parallel arrangement yields maximum efficiency and economy.The system is fully-automatic and heats 600 l. of water up to 55°C in winter months at Roorkee. Heat losses at night may be compensated for, if required, by an auxiliary electric heater provided in the storage tank and controlled by a low-cost radiation-sensing device. A simple electric circuit controls the tank mean temperature of water in the storage tank.Required collector areas based on meteorological observations for various water capacities and water temperatures are given for several Indian cities.     

Year round performance studies on a built-in storage type solar water heater at Jodhpur, India

An improved solar water heater (capacity 901) made up of a 112×80×10 cm rectangular tank which performs the dual function of absorbing heat and storing the heated water has been designed and a prototype tested in Jodhpur. The performance tests carried out at the Central Arid Zone Research Institute, Jodhpur, indicate an efficiency factor reaching as high as 70 per cent. The year-round performance tests show that this heater can supply 901 of water at a mean temperature of 50 to 60°C in winter and 60 to 75°C in summer (measured at 4:00 p.m.). The performance tests also indicate that sufficient hot water can be obtained in the early morning if the heater is covered with an insulation blanket overnight or if the hot water is stored in an insulated tank.A performance equation for this type of heater, where the inputs are the solar intensity, ambient air temperature and geometry and material specifications of the heater, has also been developed. With this performance equation the optimum gap depth, i.e. the distance between upper and lower plate of the heater, has been found to be 10·0 cm.     

Low-profile solar water heaters: The mirror booster problem revisited

We discuss the use of a plane mirror, attached to the elevated storage tank of a thermosiphon solar water heater, in order to lower the total profile of the system. The discussion includes a critical review of some of the simplifying assumptions that have been made by previous authors who have proposed the use of mirror boosters in order to enhance the energy collection of flat-plate receivers.     

Thermal performance of integrated collector storage solar water heater with corrugated absorber surface

An investigation is reported of the thermal performance of an integrated solar water heater with a corrugated absorber surface. The thermal performance of the rectangular collector/storage solar water heater depends significantly on the heat transfer rate between the absorber surface and the water, and on the amount of solar radiation incident on the absorber surface. In this investigation, the surface of the absorber is considered to be corrugated, with small indentation depths, instead of plane. The modified surface has a higher characteristic length for convective heat transfer from the absorber to the water, in addition to having more surface area exposed to solar radiation. The corrugated surface based solar water heater is determined to have a higher operating temperature for longer time than the plane surface. It means during the operation of water heater, more solar energy is converted into useful heat. However, this modification has reduces the efficiency of the system marginally.     

A solar water heater with a built-in latent heat storage

In this paper, we have investigated the performance of a novel built-in storage type water heater containing a layer of PCM-filled capsules at the bottom. The PCM layer is introduced with a view of getting hot water during off-sunshine hours. The moving solid-liquid boundary layer problem for the PCM material is simplified to a stationary boundary layer problem, and the effect of latent heat is included in the specific heat by replacing the semi-melted PCM by a fictitious solid. The performance of the water heater is then predicted analytically for two depths of PCM and for different flow rates, both constant and intermittent. The case of sudden withdrawal of water over very short periods is also studied.     

新型整体式太阳能热水器的优化设计和实验研究

本文的新型焖晒式热水器主要有两个特点 :一是水箱采用倒三角形且水平横向长纵向短的设计方式 ,以减少水箱中的死水和承压问题 ;二是在玻璃盖板和吸热面之间覆盖透明V形槽或透明蜂窝 ,该种透明隔热构件既有较高的太阳透过率又有较好的隔热性能 ,虽然减少了到达吸热面上的太阳辐射能 ,但保温性能提高了 1～ 2倍。该种热水器热效率接近真空管热水器 ,但成本低 ,且不怕冻冰 ,是我国大部分地区全年可用的低成本高效率的热水器。     

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.     

Effect of thermal trap on the performance of an underground shallow solar pond water heater

A simple analysis of an underground shallow solar pond water heater has been presented. The effect of a thermal trap at the top of the system has also been incorporated in the analysis. Using the model, the effect of various system parameters, viz. thermal trap thickness, heat capacity of water mass, flow rate and duration of flow rate have been studied in detail. Numerical calculations have been made for a typical winter day at New Delhi (India). It is concluded that the system with thermal trap gives better performance in comparison with a system with a movable insulation system.     

Numerical analysis of charging and discharging performance of an integrated collector storage solar water heater

This work aims to evaluate the energy and the exergy performance of an integrated phase change material (PCM) solar collector with latent heat storage in transient conditions. A theoretical model based on the first and the second laws of thermodynamics is developed to predict the thermal behaviour of the system. The effect of natural convection on heat during the melting process is taken into account using an effective thermal conductivity. Influence of PCM thicknesses on the melt fraction, on the energy stored and on the exergy destroyed are studied during charging and discharging processes. Results indicate that the complete melting time is shorter than the solidification time. The latent heat storage system increases the heating requirements at night and reduces the exergy efficiency.     

Effect of a thermal trap on the performance of an inexpensive underground water heater collector with storage

We present an analytical model of a ground collector which uses thermal trap material instead of glazing on its top. The effects of different parameters e.g., the depth of the heat retrieval plane, thickness of the thermal trap material and the use of various materials such as concrete, sand and bricks above the heat retrieval plane are studied. The proposed system is compared with a conventional system.     

Performance of a non-metallic unglazed solar water heater with integrated storage system

The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 l. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day’s test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 °C has been achieved for a clear day operation at an average solar radiation level of 700 W m⁻² and ambient temperature of 30 °C. The decrease of water temperature with and without the thermal diode is 10 and 20 °C, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60–63 °C were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m⁻² and ambient temperature of 31 °C.     

Experimental investigation of a domestic solar water heater with solar collector coupled phase-change energy storage

Phase change materials (PCMs) have good properties such as high thermal capacity and constant phase change temperature. Their potential use in solar energy storage is promising. Tests of exposure and constant flow rate are performed to investigate the thermal performance of a domestic solar water heater with solar collector coupled phase-change energy storage (DSWHSCPHES). Due to the low thermal conductivity and high viscosity of PCM, heat transfer in the PCM module is repressed. The thermal performance of the DSWHSCPHES under exposure is inferior to that of traditional water-in-glass evacuated tube solar water heaters (TWGETSWH) with an identical collector area. DSWHSCPHES also performs more efficiently with a constant flow rate than under the condition of exposure. Radiation and initial water temperature have impacts on system performance; with the increase of proportion of diffuse to global radiation and/or initial water temperature, system performance deteriorates and vice versa.