Performance of an inexpensive constant flow solar collector/storage system in ground

This paper presents an analysis of the performance of an inexpensive constant flow solar collector/storage system, which has been validated by experiments. the system consists of a network of pipes buried in the ground, the top surface of the ground being blackened by black board paint spray and suitably glazed. the heat can be extracted by flow of fluid in the pipes at a constant flow rate. It is seen that for a 7 cm depth of the plane of heat retrieval and 81/min flow rate of water, the collection efficiency of the system is 20.0 per cent. the efficiency increases with flow rate and decreases with the depth of the plane of heat retrieval.     

Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.     

Heat retaining integrated collector/storage solar water heaters

An integrated collector/storage solar water heater (ICSSWH) that can significantly reduce heat loss to ambient during non-collection periods has been developed. Two thirds of the ICS vessel is mounted within a concentrating cusp, McIntire ‘W’ modified concentrator and incorporates an inner heat retaining vessel. The remaining upper 1/3 of the vessel is situated outside the reflector cavity and is heavily insulated. Over 60% of the thermal energy stored within the total vessel, and up to 67% of that in the upper immediate draw-off region can be retained over a 16-h non-collection period. Results of an experimental analysis of this design and a comparison with a standard ICS design are presented.     

Heat transfer analysis of a flat-plate solar energy collector

A model is developed for the heat transfer in a flat plate solar collector with a rectangular channel for water or air flow. This 2-dimensional geometry offers the maximum area of contact between the fluid and the collecting surface exposed to the Sun. The analysis yields temperature and heat flow distributions in both dimensions of the collector. Thermal boundary layer development is investigated. Overall efficiencies are calculated for uniform solar heat influx with variable heat losses from the plate. The thermosyphonic effect, due to natural convection, is evaluated and the collector's geometry optimized with respect to this effect.     

Integrated collector storage solar systems with asymmetric CPC reflectors

New types of ICS solar systems were designed and outdoor tests of experimental models were performed. The systems consist of single cylindrical horizontal water storage tanks placed inside stationary truncated asymmetric CPC reflector troughs of different design. We used high emittance absorber surface, low cost curved reflectors, iron oxide glazing and thermal insulation at the non illuminated tank surfaces, aiming towards cost effective ICS systems with satisfactory heat preservation during the night. Four experimental models of different designs were constructed and tested to determine their performance regarding their mean daily efficiency and thermal losses during the night. The new ICS systems were compared to an ICS system with symmetric CPC reflectors of similar construction and dimensions and also to a typical Flat Plate Thermosiphonic Unit (FPTU). Test results showed that the ICS systems with asymmetric CPC reflectors present almost the same mean daily efficiency and better preservation of hot water temperature during the night, compared to the ICS system with the symmetric CPC reflectors. The comparison with the FPTU system confirmed the satisfied daily operation of all ICS systems and their moderate storage heat preservation during the night. Theoretical results showed acceptable thermal performance of all ICS systems regarding annual operation.     

Heat transfer analysis of receiver for large aperture parabolic trough solar collector

Parabolic trough solar collector (PTSC) is one of the most proven technologies for large‐scale solar thermal power generation. Currently, the cost of power generation from PTSC is expensive as compared with conventional power generation. The capital/power generation cost can be reduced by increasing aperture sizes of the collector. However, increase in aperture of the collector leads to higher heat flux on the absorber surface and results in higher thermal gradient. Hence, the analysis of heat distribution from the absorber to heat transfer fluid (HTF) and within the absorber is essential to identify the possibilities of failure of the receiver. In this article, extensive heat transfer analysis (HTA) of the receiver is performed for various aperture diameter of a PTSC using commercially available computational fluid dynamics (CFD) software ANSYS Fluent 19.0. The numerical simulations of the receiver are performed to analyze the temperature distribution around the circumference of the absorber tube as well as along the length of tube, the rate of heat transfer from the absorber tube to the HTF, and heat losses from the receiver for various geometric and operating conditions such as collector aperture diameter, mass flow rate, heat loss coefficient (HLC), HTF, and its inlet temperature. It is observed that temperature gradient around the circumference of the absorber and heat losses from the receiver increases with collector aperture. The temperature gradient around the circumference of the absorber tube wall at 2 m length from the inlet are observed as 11, 37, 48, 74, and 129 K, respectively, for 2.5‐, 5‐, 5.77‐, 7.5‐, and 10‐m aperture diameter of PTSC at mass flow rate of 1.25 kg/s and inlet temperature of 300 K for therminol oil as HTF. To minimize the thermal gradient around the absorber circumference, HTFs with better heat transfer characteristics are explored such as molten salt, liquid sodium, and NaK78. Liquid sodium offers a significant reduction in temperature gradient as compared of other HTFs for all the aperture sizes of the collector. It is found that the temperature gradient around the circumference of the absorber tube wall at a length of 2 m is reduced to 4, 8, 10, 13, and 18 K, respectively, for the above‐mentioned mass flow rate with liquid sodium as HTF. The analyses are also performed for different HTF inlet temperature in order to study the behavior of the receiver. Based on the HTA, it is desired to have larger aperture parabolic trough collector to generate higher temperature from the solar field and reduce the capital cost. To achieve higher temperature and better performance of the receiver, HTF with good thermophysical properties may be preferable to minimize the heat losses and thermal gradient around the circumference of the absorber tube.     

Heat transfer coefficients in two-phase flow for mixtures used in solar absorption refrigeration systems

This paper describes the experimental results obtained on the heat transfer in saturated nucleate boiling of refrigerant mixtures used in solar absorption refrigeration systems flowing upward in a uniformly heated vertical tube. The mixtures analysed were water/ammonia, ammonia/lithium nitrate and water/lithium bromide. The concentration range for the water/ammonia was from 38 to 48 wt%, for the ammonia/lithium nitrate was from 38 to 48 wt% and for the water/lithium bromide was from 48 to 58 wt%. It was observed that the average heat transfer coefficients increased for the mixtures with an increase in the quality, the heat flux and the solution concentration. Comparing the values of the average heat transfer coefficients for the three mixtures, it was observed that the highest values were obtained for the ammonia/water mixture.     

Theoretical storage capacity for solar air pretreatment liquid collector/regenerator

A new liquid regeneration equipment—solar air pretreatment collector/regenerator for liquid desiccant cooling system is put forward in this paper, which is preferable to solution regeneration in hot and moist climate in South China. The equipment can achieve liquid regeneration in lower temperature. When the solution and the air are in “match” state in collector/regenerator, a match air to salt mass ratio ASMR^* is found by theoretical study in which there is the largest theoretical storage capacity SC_max. At T_r = 60 °C and X_in = 2.33 kg/kg, theoretical calculation discovers when Y_in drops from 29 to 14 g/kg, the SC_max increase 50% compared with ASMR^* being around 26–27. After two new concepts of the effective solution proportion (EPS) and the effective storage capacity (ESC) are defined, it is found by theoretical calculation that when ESP drops from 100% to 67%, ESC raises lowly, not drops and liquid outlet concentration C_str.sol increases from 40% to 49% in which its increment totals to 90%. All these data explain fully that air pretreatment liquid regeneration equipment enables to improve the performance of liquid desiccant cooling system.     

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.     

Heat retaining integrated collector storage solar water heater with asymmetric CPC reflector

A novel integrated collector storage solar water heater (ICSSWH) was designed, optically analysed and experimentally studied. The unit was based around a heat retaining ICS vessel design consisting of two concentric cylinders mounted horizontally inside a stationary truncated asymmetric compound parabolic concentrating (CPC) reflector trough. The annulus between the cylinders was partially evacuated and contained a small amount of water, which changed phase at low temperature, producing a vapour and creating a thermal diode transfer mechanism from the outer absorbing surface to the inner storage vessel surface. The absorbing outer vessel surface covered with selective absorber film and was partially exposed to solar radiation. The remaining vessel surface area (including the vessel ends) was thermally insulated to improve heat retention during the night. Curved reflectors with a high reflectance along with high transmittance glazing were also used to improve effective operation of the ICS system. The thermal behaviour of the ICS system was compared to that of a Flat Plate Thermosiphonic Unit (FPTU). The experimental results showed that the ICS system is as effective during daily operation as it is during the night. Furthermore, the thermal loss coefficient during night gives similar values between the ICS system and FPTU.     

Integrated collector storage solar water heaters

The Integrated Collector Storage Solar Water Heater (ICSSWH) developed from early systems comprised simply of a simple black tank placed in the sun. The ICSSWH, by its combined collection and storage function suffers substantial heat losses to ambient, especially at night-time and non-collection periods. To be viable economically, the system has evolved to incorporate new and novel methods of maximising solar radiation collection whilst minimising thermal loss. Advances in ICS vessel design have included glazing system, methods of insulation, reflector configurations, use of evacuation, internal and external baffles and phase change materials.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in the ground; the ground is covered with a glazed thermal trap. The heat is extracted by means of a flow of liquid in such a way that the collection temperature remains constant. An expression has been derived for the periodic rate at which heat can be retrieved to keep the collection temperature constant. Numerical calculations for a typical cold day in New Delhi predict that, for a collection temperature of 30°C and for a trap thickness of 0·03 m, the maximum possible integrated collection efficiency is 25·7 per cent.     

Performance of a thermal trap solar collector/storage system

This paper presents an analysis of a novel solar collector/storage system consisting of a network of pipes buried in a mass of sand; the sand is covered with a glazed thermal trap. The heat can be extracted by flow of fluid in the pipes at a constant flow rate. An expression has been derived for the periodic rate at which useful heat can be collected, keeping the flow rate constant. Numerical calculations for a typical cold day in Delhi predict that the collection efficiency of the system is about 50% for flow rate of 10 kg/h. The efficiency increases with thermal trap thickness and with flow rate.     

Semi-transparent solar collector window systems

Semi-transparent window solar collectors which can be integrated with opaque wall collectors for space heating and ventilation are described for buildings containing large quantities of glass. Simpler, inexpensive, retractable systems are also described. Such systems can have rapid impact on fuel savings. The achievement of 20 per cent grey-scale light transmission combined with 75 per cent thermal utilization of the total solar energy seems feasible. The collector system works by means of semi-transparent thin absorber and reflector coatings on double-glaze windows. Vents at the top and bottom of the double-coated window system permit convective flow of solar-heated air currents for space heating on cold sunny days, convective room ventilation on warm sunny days, and thermal insulation in the absence of sunshine. An account is given of experimental work on sunlight-absorbing and reflecting materials and coatings for implementation of the window systems. These metallic, (alloy or bilayer) and semi-conductor coatings also have applications to Schottky barrier and conventional solar cells and coatings for solar thermal collectors.     

Heat transfer coefficient and thermal losses of solar collector and Nusselt number correlation for rectangular solar air heater duct with longitudinal fins hold under the absorber plate

An experimental investigation has been carried out for a series of system and operating parameters in order to analyze the effect of mass flow rate on heat transfer and Nusselt number characteristics in solar air heater. Experiments are performed at different air mass flow rates; varying from 0.012 to 0.016 kg/s, about hot summer days of Mai 2012. Hourly values of global solar radiation and some meteorological data (temperature, wind speed, relative humidities, etc.) for measuring days are obtained from the Biskra city of Algeria. The experiments encompassed the flow Reynolds number in the range 965.48–1301.4. Longitudinal fins were used inferior the absorber plate for an increase the heat exchange and render the flow fluid in the channel uniform. The effects of mass flow rate of air on the outlet temperature, Nusselt Number, Reynolds Number, Prandtl Number, the heat transfer in the thickness and length of the solar air collector were studied. For this effect was have created a new correlation correspondent of solar air collector with using fins it was written Nu = κ₀Re^1.36Pr^?0.68exp(0.342m)h ^[?0.018Pr].     

Heat transfer coefficients in two phase flow for the water/lithium bromide mixture used in solar absorption refrigeration systems

This paper describes the experimental results obtained from the heat transfer in saturated nucleate boiling for the water/lithium bromide mixture flowing upward in a uniformly heated vertical tube, which is the generator of a solar absorption refrigeration system. The concentration range for the mixture was from 48 to 56 wt.% Plots of local and average heat transfer coefficients are shown against solution concentration, heat flux and the temperature difference between the wall tube and the fluid. It was observed that the average heat transfer coefficients increased for the mixture with an increase of the heat flux and with the decrease of the solution concentration and the temperature difference. The average heat transfer coefficients varied from 1.0 to 4.0 kW/m² °C.     

Honeycomb solar pond collector and storage system

An analysis of a honeycomb-stabilized, saltless solar pond as a solar energy collector and long term (spanning seasons) storage system is presented. The solar pond is considered with a nonconvective zone made up of an oil layer and air honeycomb configuration. A heat flow model is developed using the two loss mechanisms (conduction and radiation). The efficiency of heat collection and the storage characteristics of the system are excellent for hot water production and process heat applications.     

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.     

Techno-economic appraisal of an integrated collector/storage solar water heater

Integrated collector/storage solar water heaters, due to their simple compact structure and inherent freeze protection, offer a promising approach for solar water heating in colder climates. Such a system, designed specifically for application at a Northern latitude, has been developed incorporating a heat retaining storage vessel mounted within a concentrating cusp reflector supported by a novel exo-skeleton framework. The performance was determined experimentally under real operational conditions in the Northern Irish climate. A detailed cost analysis is presented and payback periods, substituting different local fuel/power sources, determined.     

A new method for the measurement of solar collector time constant

A new test method about the time constant of the solar collector has been presented in this paper. It is simple and has been validated through experiments. With the new method it is not necessary to adjust the inlet temperature of the transfer fluid as closely as possible to the ambient air temperature. Also, it is not necessary to know the characteristic parameters of the collector in advance. The model used in the paper is a first order system model, as in most cases. The experimental data obtained from the test of solar collector time constant shows that the solar collector is not a strictly first order system. A criterion is proposed to decide whether the system is a first order system or not and the resemblance of the system to the first order system.