A method of determining the time constant of the flat-plate solar collector

Analysis of the flat-plate solar water heating system indicates that the transient response of the system is determined by the time constants of the solar collector and storage tank. The time constants are dependent on the thermal capacitances and heat loss coefficients of the collector and storage tank and on the capacitance rate of the circulating liquid. This paper presents a method of determining the time constants of the system from test results on a solar simulator.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

Development of advanced solar assisted drying systems

The Solar Energy Research Group in the Universiti Kebangsaan Malaysia has been set-up more than two decades ago. One of the activities is in the field of solar thermal process, particularly in development of solar assisted drying systems. Solar drying systems technical development can proceed in two directions. Firstly simple, low power, short life, and comparatively low efficiency-drying system. Secondly, the development of high efficiency, high power, long life expensive solar drying system. The group has developed four solar assisted drying systems namely (a) the V-groove solar collector, (b) the double-pass solar collector with integrated storage system, (c) the solar assisted dehumidification system for medicinal herbs and (d) the photovoltaic thermal (PVT) collector system. The common problems associated with the intermittent nature of solar radiation and the low intensities of solar radiation in solar thermal systems can be remedied using these types of solar drying systems. These drying systems have the advantages of heat storage, auxiliary energy source, integrated structure control system and can be use for a wide range of agricultural produce.     

A new type of solar water heater

This investigation reports a new type of solar water heating system without water pipes on the collector surface or a separate storage tank. The water to be heated continuously flows perpendicularly from an upper transparent cover to a porous absorber and is stored in a small volume beneath this assembly. Three different systems were designed, manufactured and tested but only one proved to be successful; this design indicated higher thermal efficiency compared to conventional collectors at high flow rates whereas at low flow rates the opposite is true.     

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.     

Experimental investigation and performance analysis on a solar adsorption cooling system with/without heat storage

A solar adsorption cooling system which can be switched between a system with heat storage and a system without heat storage was designed. In the system with heat storage, a heat storage water tank was employed as the link between the solar collector circulation and the hot water circulation for the adsorption chillers. However, the heat storage water tank was isolated in the system without heat storage, and the hot water was directly circulated between the solar collector arrays and the adsorption chillers. It was found that the inlet and outlet temperatures for the solar collector arrays and the adsorption chillers in the system without heat storage were more fluctuant than those of the system with heat storage. Also found was that the system with heat storage operated stably because of the regulating effect by the heat storage water tank. However, under otherwise similar conditions, the cooling effect of the system without heat storage was similar to that of the system with heat storage. Compared with the system with heat storage, the system without heat storage has the advantages of higher solar collecting efficiency as well as higher electrical COP.     

Design and thermal performance of an ICS solar water heater based on three parabolic sections

An integrated collector storage (ICS) consisting of a single cylindrical horizontal tank placed in a reflector composed of three parabolic branches is designed and geometric characteristics are determined. The suggested design aims to cover the need of hot water of a family composed of four persons. Based on this target, its geometric characteristics: reflector geometry, aperture, reflector length, are derived.The comparison between this system and two other systems of solar water heater, composed of a storage tank with asymmetrical CPC and symmetrical CPC, shows important thermal performances despite the simplicity and the little cost of our collector. The first experimental results are given and its comparison with the theoretical results demonstrates a good agreement.     

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 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.     

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 studies of a new solar water heater system using a solar water pump

The research goal was to develop a new solar water heater system (SWHS) that used a solar water pump instead of an electric pump. The pump was powered by the steam produced from a flat plate collector. Therefore, heat could be transferred downward from the collector to a hot water storage tank. The designed system consisted of four panels of flat plate solar collectors, an overhead tank installed at an upper level and a large water storage tank with a heat exchanger at a lower level. Discharge heads of 1, 1.5 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 7–14 kPa. It was found that water circulation within the SWHS ranged between 12 and 59 l/d depending on the incident solar intensity and system discharge head. The average daily pump efficiency was about 0.0014–0.0019%. Moreover, the SWHS could have a daily thermal efficiency of about 7–13%, whereas a conventional system had 30–60% efficiency. The present system was economically comparable to a conventional one.     

Technoeconomic optimization of a hybrid solar air-heating system

This paper presents a simple techno-economic model for a hybrid solar air-heating system based on water as the storage medium. The configuration of the system consists of a conventional solar air-heater, water tank for thermal storage, a unit which adjusts the higher air temperature (during peak sunshine hours) to the required limit (by mixing fresh air) and an arrangement for providing auxiliary energy if and when required. A thermostatically controlled electric heater is assumed to be the source of auxiliary energy, in the present calculations. In order to evaluate the performance of the system using the developed model numerical calculations have been made corresponding to the climate of Delhi, India. The calculations have been extended to obtain the optimized values of collector area and storage mass which correspond to the minimum value of useful energy. Numerical results show that the cost of useful energy obtained for optimized values of collector area and storage mass is much less than the cost of electrical heating.     

The dependence of thermosyphonic system performance on collector aspect ratio

The dependence of a standard thermosyphonic solar system's performance on collector aspect (length to width) ratio is investigated. Numerical simulation of the system's characteristics is conducted for systems with the same overall collector area but three aspect ratios. In all cases, spacing between collector tubes was kept constant to eliminate fin efficiency considerations. The systems were simulated for typical summer and winter days. Results of the investigation reveal that overall performance characteristics (like bulk efficiency or average tank temperature) as well as stratification in the tank and flow rates depend only slightly on the aspect ratio. The practical conclusion is that only economical considerations and not energetical ones should be applied in designing the aspect ratio of a thermosyphonic solar system.     

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.     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.     

Temperature stratification from thermal diodes in solar hot water storage tank

In this brief note, we have experimentally measured the temperature stratification in a solar hot water storage tank resulting from a simulated solar heating load. Various modifications using a double chimney device that acts as a thermal diode were examined with the intent of maximizing temperature stratification. The greatest stratification was seen with a unique thermal diode arrangement named the express-elevator design, so-called for the direct hot water path from the bottom third of the tank to the top third.     

Exergy analysis of integrated photovoltaic thermal solar water heater under constant flow rate and constant collection temperature modes

In this communication, an analytical expression for the water temperature of an integrated photovoltaic thermal solar (IPVTS) water heater under constant flow rate hot water withdrawal has been obtained. Analysis is based on basic energy balance for hybrid flat plate collector and storage tank, respectively, in the terms of design and climatic parameters. Further, an analysis has also been extended for hot water withdrawal at constant collection temperature. Numerical computations have been carried out for the design and climatic parameters of the system used by Huang et al. [Huang BJ, Lin TH, Hung WC, Sun FS. Performance evaluation of solar photovoltaic/thermal systems. Sol Energy 2001; 70(5): 443–8]. It is observed that the daily overall thermal efficiency of IPVTS system increases with increase constant flow rate and decrease with increase of constant collection temperature. The exergy analysis of IPVTS system has also been carried out. It is further to be noted that the overall exergy and thermal efficiency of an integrated photovoltaic thermal solar system (IPVTS) is maximum at the hot water withdrawal flow rate of 0.006 kg/s. The hourly net electrical power available from the system has also been evaluated.     

A comparison of solar absorption system configurations

Solar thermal driven cooling systems for residential applications are a promising alternative to electric compression chillers, although its market introduction still represents a challenge, mainly due to the higher investment costs. The most common system configuration is an absorption chiller driven by a solar thermal system, backed up by a secondary heating source, normally a gas boiler. Heat storage in the primary (solar) circuit is mandatory to stabilize and extend the operation of the chiller, whereas a cold storage tank is not so common.This paper deals with the selection of the most suitable configuration for residential cooling systems with solar energy. In Spain, where cooling needs are usually higher than heating needs, the interest of a reversible heat pump as auxiliary system and a secondary cooling storage are analyzed.A complete TRNSYS model has been developed to compare a configuration with just hot storage (of typical capacity 40 L/m² of solar collector surface) and a configuration with both, hot and cool storages. The most suitable configuration is very sensible to the solar collector area. As the collector area increases, the advantages of a cool storage vanish. Increasing the collector area tends to increase the temperature of the hot storage, leading to higher thermal losses in both the collector and the tank. When the storage volume is concentrated in one tank, these effects are mitigated. The effect of other variables on the optimal configuration are also analyzed: collector efficiency curve, COP of the absorption chiller, storage size, and temperature set-points of the chillers.     

Experimental study of CPC type ICS solar systems

Extensive experimental study on solar water heaters, which were developed in our laboratory, is presented. These solar devices are integrated collector storage (ICS) systems with single horizontal cylindrical storage tank properly placed in symmetric CPC type reflector trough. In this paper we study ICS solar systems, which differ in storage tank diameter and correlate their thermal performance and the ratios of the stored water volume per aperture area and also per total external surface area. Based on the results of this study and aiming to achieve improved ICS systems, we considered an effective tank diameter and we extracted by outdoor tests the performance of a number of experimental models differing in the absorbing surface, reflector and transparent cover. We calculated the mean daily efficiency and the thermal loss coefficient during night of each system combination. In addition, 24 h and four days operation diagrams of the variation of water temperature of the studied ICS systems are compared with the corresponding diagrams of two flat plate thermosiphonic units with mat black and selective absorbing surface, respectively. The experimental results show that ICS system with selective absorbing surface, high transmissivity of the transparent cover and high reflectance of its reflector surface performs efficiently enough, both during the day and night operation, approaching the thermal performance of the corresponding thermosiphonic unit of flat plate collector with selective absorber.