Transient behaviour of collector/storage solar water heaters for generalised demand patterns

This communication presents a simple transient model for predicting the thermal performance of collector/storage solar water heaters for generalised demand patterns. These heaters consist of either (i) an insulated rectangular metallic tank whose top surface is blackened and suitably glazed (i.e. a built-in storage solar water heater) or (ii) an insulated open shallow tank with black bottom.inner sides and a glass plate at the surface in contact with the water (i.e. a shallow solar pond water heater). The time dependence of the water temperature for the withdrawal of hot water from the system at constant flow rates constantly or intermittently has been explicitly evaluated. Numerical results for the operation of the system in industrial and community service applications are discussed.     

Physics of shallow solar pond water heater

This communication presents a theoretical analysis of a shallow solar pond water heater, which is in good agreement with the experiments of Kudish and Wolf (1979) and the authors. the heater consists of an insulated metallic rectangular tank with black bottom and sides and a transparent cover at the top. After the collection of solar energy during sunshine hours the heater stores a substantial amount of heat because the top glass cover is covered by an adequate insulation in the night. Analytical expressions for the transient rise of temperature of water in the tank have been derived taking into account the appropriate heat transfer processes during day and night. These experimental results as well as those of Kudish and Wolf (1979) have been found to be in good agreement with the theory presented in this paper. the effects of one more glass cover on the top, and of the thickness of the bottom and side insulation and tank depth on the water temperature have also been studied.     

Performance study of forced circulation solar water heaters using packed-bed collectors

A performance study of forced circulation solar water heaters using packed-bed solar collectors is presented. Iron chips, gravel and stones have been used as packing materials. Thermal energy stored in the tank, system overall efficiency and pay-back capital for these solar water heaters are compared with those for solar water heaters using a plane collector. It is observed that the performance of the solar water heater is improved appreciably by packing its collector with packing material. A solar water heater using an iron chip, packed-bed collector shows the best performance.     

Shallow solar pond water heater: An analytical study

This paper presents a simplified analytical model to predict the performance of a shallow solar pond water heater which combines both collection and storage in a single unit. Essentially, the water heater consists of a metallic (G.I.) tray, whose inner and bottom surfaces are blackened, and is covered with a transparent sheet at the top; the sides and bottom surfaces of the assembly are well insulated. The unit stores a substantial amount of heat for the next day morning's use when the top cover is insulated during the off-sunshine hours. The performance of the water heater, both during the day and night time, can satisfactorily be predicted by this theoretical model. The theoretical calculations are found to be in good agreement with the experimental observations reported earlier.     

Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters

The thermal performance of transparently insulated integrated-collector-storage solar water heaters is investigated theoretically as well as experimentally for a comparative study of cover systems having transparent insulation materials devices placed between the top glazing and the absorber. The data on solar transmittance, heat loss reduction characteristics and solar collector-storage efficiencies of various configurations is generated for the system performance comparisons. These hot water systems exhibit average (diurnal basis) solar collection and storage efficiencies in the range of 20–40% at a collection temperature of 40–50°C. The performance of water heaters with cover system having absorber-perpendicular configuration of TIM excel over absorber-parallel TIM covers. The effect of variation in the temperature of heat collected and cost of cover systems on the performance comparisons is also discussed.     

Performance study of thermosyphonic circulation solar water heaters using packed bed collectors

In the present investigation the performance behaviour of thermosyphonic circulation solar water heaters using packed bed collectors has been analysed. Iron chips, gravels and stones have been used as packing materials. Average tank water temperature, collector as well as system efficiency and mony pay-back for these packed bed solar water heaters are compared with those for solar water heater using a plane collector. Experimental results reveal that the performance of solar water heater improves appreciably by packing its collector with packing material. Among the packed-bed solar water heaters tested the iron chips packed-bed solar water heater gives the overall best performance.     

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.     

Comparisons of deep well and insulated shallow earth storage of solar heat

Four techniques for storing solar heat in earth are described and their costs are compared by a hypothetical example. They include heat storage by: (a) deep well; (b) deep well with a storage membrane; (c) insulated shallow earth heat exchange bed; and (d) insulated shallow earth tube exchanger. The cost comparison obtains optimal design parameters for each storage method and optimal surface area for an attached solar collector. Heat loss equations for insulated shallow earth storage are obtained by electrolytic tank models. Heat loss for deep well storage is derived by superposition of radial flow of water and diffusion of heat.     

Optimisation of minimum backup solar water heating system

Current flat plate solar water heaters overproduce slightly in summer and have poor performance in winter at the time of maximum load. They use an expensive absorber plate over the entire absorbing aperture of the collector and fail to use the backside of the absorber. They often have under insulated tanks and are not optimised as integrated systems. This paper describes a design approach taken to use existing commercial flat plate absorber and tank components in a new way to maximise solar contribution and minimise material usage in the construction of the system. The design criterion used is not maximum peak efficiency, but minimum annual backup energy supplied to the system to meet an annual load. This corresponds to meeting a minimum greenhouse emissions requirement in both invested pollution during manufacture and pollution from backup energy supplied. Two new designs are shown which allow the solar fraction of systems to be increased to approximately 80–90% in Sydney Australia using a standard model of domestic hot water usage specified in Australian Standard AS4234. Pollution from fuel use drops to as little as 40% of that of conventional flat plate solar water heaters. These new designs use one absorber plate instead of two and a smaller and better insulated tank. Comparisons of solar fraction are evaluated for a range of climatic conditions. An important insight is that with such a performance optimised system the ultimate solar fraction is limited by occasional long duration cloud cover at the site of installation and making the system larger only increases dumped energy, not utilisable energy. Technical efficiency improvements only reduce the required collector area. However, some additional backup fuel reductions can be made through manual control of backup energy use, because this allows finer control of backup relative to real demand. Pollution from backup fuel usage may be able to be reduced to 1/4 that of current flat plate solar water heaters.     

Performance of experimental solar water heaters in Australia

Seven experimental solar water heaters were installed at C.S.I.R.O. laboratories throughout Australia in order to gain field experience and performance data for various localities. Each heater included an insulated 70-gal hot water storage tank with a built-in electric booster and two solar absorbers of total active area of 45 sq ft.

Each morning approximately 45 gal of water at a temperature of about 135°F were discharged from the tank. Average monthly values for a 12-month period are given for the daily electric power consumption and the solar contribution. Mean yearly contribution of the solar energy under these conditions was from 60 to 80 per cent of the total energy required, depending on the district in which the heaters were located. In order to determine the extent to which the results are typical, a comparison is given, for some of the districts, of the sunshine hours recorded during the test period, with the nominal 30-year average.     

ICS solar systems with horizontal (E–W) and vertical (N–S) cylindrical water storage tank

The use of a horizontal cylindrical water storage tank contributes to pressure resistant, low height and efficient ICS solar systems. These systems can satisfactorily achieve water heating when the cylindrical storage tank is combined with stationary CPC or involute type curved reflectors. The diameter of the cylindrical storage tank determines the length of the reflectors, the system depth and the ratio of the stored water per aperture area. In these solar systems the storage tank can be partially thermally insulated to suppress thermal losses from it to the ambience. We constructed four experimental models with truncated symmetric CPC reflectors, two with 90° and other two with 60° of acceptance angle, half of them without and half with a 1/4 thermally insulated storage tank cylindrical surface. In addition, we constructed two ICS systems with involute reflectors, with acceptance angle 180°, one without and the other one with a 1/4 thermally insulated storage tank. The six ICS systems were tested under the same weather conditions and without water drain, to determine their stored water temperature variation, mean daily efficiency and thermal losses during night. The results showed that CPC reflectors contribute to efficient operation of systems day and night, while involute reflectors mainly to the water heat preservation during night.     

A novel underground solar water heater

This communication presents the performance of a novel solar water heater which consists of a rectangular tank (110 × 80 × 10 cm) made up of G.I. sheet and is provided with inlet and outlet pipes. The tank is buried underground; the top surface of the ground is blackened and glazed so that the surface acts as an absorber of solar radiation while the tank as well as the sorrounding earth serves as the storage system. During off-sun shine hours the top surface is covered by a layer of insulation so as to prevent upward loss. An analytical model has been presented to predict the performance of such a system.     

Outlook for solar water heaters in Taiwan

The share of indigenous energy supply continuously decreases over the last two decades in Taiwan. The development and use of renewable energy sources and technologies are becoming vital for the management of energy supply and demand. For promotion of solar water heaters, the incentive programs were firstly initiated in the period of 1986–1991 and re-initiated from 2000 to the present. These programs create an economic incentive for the end users and have a drastic effect on the popularization of solar water heaters. To further promote solar water heaters during the current incentive program period, several key factors are addressed. In addition to the cost of solar water heaters and energy price index, the potential market of solar water heaters in Taiwan is associated with the climatic conditions, population structure, urbanization, building type of housing and status of new construction.     

Forced versus natural circulation solar water heaters: A comparative performance study

An experimental study has been carried out to compare the performance of natural and forced circulation domestic solar water heaters. Several measurments have been made for the two cases which included; the collector water inlet and outlet temperatures, the mass flow rate, the tank temperature, the ambient temperature and the solar insolation. The main parameters for the solar collector are calculated for the natural and forced circulation systems. These included; the top, back, and overall loss coefficients, the heat removal factor, the efficiency factor, the useful energy gain and the instantaneous efficiency. The comparison showed that the efficiency of the forced circulation system could be 35 to 80% higher.     

超细纤维保温水箱与聚氨酯保温水箱热性能比较

超细生态纤维是一种对生物体和环境无害的无机非金属材料。本文通过对该种纤维保温的水箱与目前太阳热水器常用的聚氨酯发泡保温的水箱的热性能进行研究后认为，在同样水箱结构下，两种水箱热性能几乎相当。但生态纤维的耐高温、易降解及制做过程无毒的特性是聚氨酯所不可比拟的。     

Performance of solar water heaters with narrow mantle heat exchangers

This paper evaluates the performance of narrow-gap vertical mantle heat exchangers with a two-pass arrangement for use in pumped-circulation solar water heaters. Both measured mantle side and tank side heat transfer correlations have been developed and implemented in a TRNSYS model of a complete solar water heater incorporating this type of heat exchanger. Predictions of the annual solar contribution for mantle-tank systems are compared to direct-coupled systems. The direct-coupled systems are found to provide slightly higher annual energy saving than mantle-tank systems for standard domestic hot water demand in Australia. However, the reduction in performance is outweighed by the benefit of freeze protection provided by incorporating a collector loop heat exchanger in the system.     

Solar and geothermal heating and cooling of the European Centre for Public Law building in Greece

The European Centre for Public Law in Legraina near Athens in Greece is heated and cooled by a combined solar and geothermal system. The main components of the system are a saline groundwater supplying well, water storage tank for 6 h autonomy, inverter for regulating geothermal flow, heat exchanger, two electrical water source heat pumps placed in cascade, fan coils, air handling units, as well as solar air collectors for air preheating in winter. In addition, hot water is supplied to the building hostel by solar water heaters. Monitoring of the energy system during heating showed excellent energy efficiency and performance.     

Solar assisted biogas digesters I: Thermal analysis

A thermal analysis has been made of a number of viable solar systems, namely (i) a solar canopy, (ii) a shallow solar pond (SSP) water heater, (iii) insulation and (iv) an SSP with a solar canopy around the digester, to boost the biogas production in conventional KVIC biogas plants during the winter months. Numerical calculations for a typical sunny winter day (17 January 1984) in the climate of New Delhi show that an SSP water heater on the top of the gas holder, covered with a solar canopy, is a good option from thermal point of view.     

ICS solar systems with two water tanks

Integrated collector storage (ICS) systems are compact solar water heaters, simple in construction, installation and operation. They are cheaper than flat plate thermosiphonic units, but their higher thermal losses make them suitable mainly for application in locations with favourable weather conditions. Aiming to the achievement of low system height and satisfactory water temperature stratification, new types of ICS systems with two horizontal cylindrical storage tanks, properly mounted in stationary CPC reflector troughs are suggested. The non-uniform distribution of solar radiation on the two absorbing surfaces is combined with the seasonal sun elevation, resulting to effective water heating. In addition, the inverted absorber concept design can be applied to ICS systems with two storage tanks. In this paper, we present the design and performance of double tank ICS solar systems, which are based on the combination of symmetric and asymmetric CPC reflectors with water storage tanks. The analytical equations of the collector geometry of all models are calculated with respect to the radius of the cylindrical water storage tank and the reflector rim angles. Experimental results for the variation of the water temperature inside storage tanks, the mean daily efficiency and the coefficient of thermal losses during night are given for all experimental models. The tests were performed without water draining and the results show that the double tank ICS systems are efficient in water temperature rise during day and satisfactory preservation of the hot water temperature during night, with the upper storage tank being more effective in performance in most of the studied models.     

Performance study of a solar water heater incorporating a bed of solid particulates

An investigation of a solar water heater with built-in storage is presented. The experimental apparatus consists, essentially, of a rectangular insulated tank packed with solid particulates. The packed system is examined with no net water flow. An equation based on a heat balance is used to predict the theoretical temperature history of the water. Comparisons demonstrate that the model agrees well with experimental values. Predictions are also made regarding the effect of using various packing materials.