Solar Air Heating Collector Research Articles

Optimized finned absorber geometries for solar air heating collectors

Optimized finned absorber geometries for solar air heating collectors

Heat transfer in flat-plate solar air-heating collectors

All energy systems involve processes of heat transfer. Solar energy is obviously a field where heat transfer plays crucial role. Solar collector represents a heat exchanger, in which receive solar radiation, transform it to heat and transfer this heat to the working fluid in the collector's channel. The radiation and convection heat transfer processes inside the collectors depend on the temperatures of the collector components and on the hydrodynamic characteristics of the working fluid Economically, solar energy systems are at best marginal in most cases. In order to realize the potential of solar energy, a combination of better design and performance and of environmental considerations would be necessary. This paper describes the thermal behavior of several types of flat-plate solar air-heating collectors.

Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates

The study investigates the more general and fundamental case when variable width absorbers are utilized instead of conventional rectangular plates in collecting incident solar radiation. The geometries considered include trapezoidal and concave parabolic absorbers having the same surface area. The effect of absorber shape on collector thermohydraulic performance has been examined for three common models with air flowing over the flat absorber, or under it, and on both sides of the absorber. The results are compared with those of rectangular absorber models over a wide range of mass flow rate and solar energy flux. Under the same operating conditions, the comparisons indicate that variable width collectors exhibit performance behavior similar to those of the constant width corresponding models, but it is accompanied by a marginal decrease in air temperature rise and collection efficiency along with a slight increase in fan power consumption. The comparisons do, thus, support the idea of employing variable width collectors in solar air heating applications, especially those which have design constraints on the investment costs and on the space available to install air collectors. For such applications, trapezoidal shape collectors are highly recommended.

Optimized finned absorber geometries for solar air heating collectors

Solar air heaters are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small heat conductivity of air. For high solar gains, an efficient thermal coupling between absorber and fluid is required, while the electrical power for the fan operation ought to be as small as possible. As heat transfer augmentation usually increases friction losses, an optimum fin geometry has to be found. For a solar ventilation air preheater, mounted on a south facing façade in Würzburg, Germany, optimized geometries have been derived. For a specific air mass flow rate of 70 kg/(m 2 h) the air gap for smooth absorbers without fins should be about 7 to 8 mm to get a maximized yearly net energy output of about 680 MJ/m 2 during the heating season from October through April. Finned absorbers perform much better. Continuous aluminum-fins, 0.1 mm thick and spaced about 6 mm apart in a 30-mm-wide air gap yield about 900 MJ/m 2 yearly net output. Offset strip fins do not show an improved performance compared to optimally spaced continuous fins, due to the larger electrical power for this geometry. However, offset strip fins yield high net energy gains for large fin spacings.

99/02112 Review of solar-energy drying systems III: low temperature air-heating solar collectors for crop drying applications: Ekechukwu, O. V. and Norton, B. Energy Conversion and Management, 1999, 40, (6), 657–667

99/02112 Review of solar-energy drying systems III: low temperature air-heating solar collectors for crop drying applications: Ekechukwu, O. V. and Norton, B. Energy Conversion and Management, 1999, 40, (6), 657–667

Review of solar-energy drying systems III: low temperature air-heating solar collectors for crop drying applications

The efficient design and construction of solar-energy air-heating collectors are critical to the overall performance of the distributed (indirect mode) and mixed-mode designs of either active or passive solar-energy crop dryers. A review of the various designs and the performance evaluation technique of flat- plate solar-energy air-heating collectors for low temperature (i.e. temperature elevations between 10°C–35°C above ambient) solar-energy crop drying applications are presented. The appropriateness of each design and the component materials selection guidelines are highlighted.

97/04998 Covered profiled steel cladding as an air heating solar collector: laboratory testing, modelling and validation: Ho, K. T. K. and Loveday, D. L. Energy and Buildings, 1997, 26, (3), 293–301

97/04998 Covered profiled steel cladding as an air heating solar collector: laboratory testing, modelling and validation: Ho, K. T. K. and Loveday, D. L. Energy and Buildings, 1997, 26, (3), 293–301

Covered profiled steel cladding as an air heating solar collector: laboratory testing, modelling and validation

Profiled steel cladding is a widely-used product in construction, and can be adapted to act as a building-integrated solar air heating collector. A mathematical model is presented of an air heating solar collector using profiled steel as the absorber and transparent multi-layer polycarbonate rigid sheeting as the cover. The model is validated by experiments conducted within a controlled laboratory environment, and the experimental results are also presented. The expected full-scale performance has been evaluated based on the assumption that the proposed solar collector is to be used as an integral part of a building wall structure situated in an urban area of the UK in winter. The evaluation leads to equivalent values for U L and for F R of approximately 4.0 W m −2°C −1 and 0.71, respectively, which provide an estimation of the performance of such a collector system in the UK.

A user oriented simulation model for deep bed solar drying of rough rice

A computer model based on mass and energy balance has been developed which incorporates a range of solar flat plate air heating collector geometries and a drying model based on the thin layer drying principle. This model will be useful in understanding the extreme complexity of the drying process which is influenced by many design parameters. The developed model will also be helpful as a decision making tool in assessing suitable solar collector design options to meet the desired drying requirements. The collector and drying models have been tested separately for a wide range of operating conditions. A description of the models and their assumptions, together with some simulation results for rough rice drying are presented in this paper.

Simulation and optimization of energy systems for in-bin drying of canola grain (rapeseed)

Energy utilization systems optimization and management strategies for in-bin drying of canola were investigated by using a validated computer simulation model and typical weather data for a prairie location in North America. The use of different energy systems, including natural gas, propane, electricity, solar energy, and combined natural gas and solar energy for drying grain within 15 days with airflow rates of 0.5–2 m 3/min t, initial grain moisture contents of 13, 16 and 19%, and three harvest dates in August, September and October, was simulated for 10% and 8% moisture contents average-dry and through-dry policies. The drying systems were optimized by considering the total annual cost of a drying system within set bounds of drying time (⪕15 days) and spoilage index (SI < 1.0). Continuous fan operation with 1.5–2 m 3/min t ambient air with about 9–26 MJ/t fan energy consumption was required to dry canola grain to 10% and 8% average-dry and through-dry moisture contents in 15 days or less August at 19% initial moisture content or less. Supplemental heat, by raising the ambient temperature by 5–10°C, maintaining the plenum temperature at 20°C and solar heating, must be applied to successfully dry the product in September and October. Solar heating for drying was found to be more cost effective than other supplemental heat systems provided a well designed flat-plate solar collector for air heating can be found for use in locations with good solar energy availability. Heating the drying air with natural gas or propane was the cost effective for situations where the use of conventional energy systems is preferable to renewable energy sources in grain drying operation.

Thermal Storage in the Walls of a Solar House

Residential winter thermal energy storage features water encapsulated into 3-in. (7.6-cm) diameter plastic pipes, mounted into conventional stud wall cavities of a house. With an air solar collector, solar-heated air can be passed through the stud cavities, heating the water. During the discharge mode, this water loses its heat directly to the house, and the radiating walls allow the residents to feel warm even at lower interior air temperatures. Empirical and theoretical component performance data are reported for the waterfall thermal energy storage unit. The interaction between a large air-heating solar collector and the waterfall thermal energy storage is considered for the winter heating mode. The collector-storage thermal integration analysis is detailed for a charging flow rate of 40 cfm per cavity. A simpler but reasonably accurate integration analysis is illustrated for 10 cfm cavity flow rate. Performance parameters indicate that the waterfall thermal energy storage approach is very compatible with a solar air heater.

Performance of air-heating collectors with packed airflow passage

This paper deals with a comparative theoretical parametric analysis of solar air-heating collectors with and without packing in the flow passage above the back plate. The air heater without packed airflow passage is the conventional type, comprising a glass cover in conjunction with an absorber and a back plate with the air flow beneath the absorber. The packed channel air heaters are of two types: the conventional type, with packing in the flow passage between the absorber and the back plate, and the other type consisting of two glass covers and a back plate with the black-painted packings (which function as absorbers) in the flow passage between the inner cover glass and the back plate. For comparing the performance, packings of different materials, shapes, and sizes with different void fractions are considered. To estimate the effective energy gain, the energy expended in pumping the air through the air heaters with different duct depths and lengths and with different airflow rates is computed.

Comparative study of air heating solar collectors

Three types of conventional solar air heater are designed such that their heat absorbing areas and the pressure drops across them are equal for equal air mass flow rates per unit collector area. The results of thermal performance tests conducted simultaneously on these collectors, under the same environmental conditions, are presented.

Performance of a solar collector with different glazing materials and their degradation under the condition prevailing in a solar collector

The effects of different glazing materials, namely double window glass (DG), single window glass (SG) and polymethyl methacrylate (PMMA), on the performance of a 2m 2 solar flat plate collector for air heating have been studied. The decrease in solar transmittance of these covers, measured between 0 and 60° angles of incidence ( θ i ), seems to be compensated by the corresponding increase in solar reflectances while the solar absorptances were not so sensitive to the variation of θ i . Under identical insolation level and ambient temperature, at air flow rates between 0.014 and 0.103 kg/s, the rise in air temperature for a 2m 2 collector with DG cover was 1 and 2°C higher than that of the collectors with SG and PMMA covers, respectively. At 0.100 kg/s (0.05 kg/s m 2), the thermal efficiencies of the collectors with DG, SG and PMMA covers were 46.0, 42.0 and 36.5%, respectively. Long term durability of the DG and PMMA systems was poor under the conditions prevailing in a solar flat plate collector. The PMMA fitted with the collector showed a degradation when it was exposed to outdoor conditions. In consequence, the decrease in solar transmittance was a maximum within 3 months, due to the formation of low molecular weight products.

Numerical simulation of the performance of an air heating solar collector

SYNOPSIS The effects of the thermal and hydrodynamics entry length for both laminar and turbulent flow regimes within an air-heating solar collector have been studied using a numerical procedure. Two internal arrangements were considered: (1) air passing on both sides of the absorber (parallel flow), and (2) use of finned and other augmented surfaces. Results of the analysis showed that the temperatures of the cover plates for the parallel flow collector were lower than if the air had passed on only one side of the absorber plate (single flow). Consequently, the heat losses were reduced and the efficiency of the parallel flow collector was increased. Also the pumping power per unit energy gained by the collector for parallel flow mode was less than that for the single flow mode. By using a finned surface, the temperatures of the cover plates were lower than for single and parallel flow and consequently the heat losses were reduced and the efficiency of the finned surface collector was increased. But in th...

Heat Exchanger Theory Applied to the Design of Water- and Air-Heating Flat-Plate Solar Collectors

A general method for the design of flat-plate solar collectors based on solar collector theory has been developed. It can be applied to both liquid- and air-heating solar collectors. The solar collector efficiency is determined by the product of the effectiveness (ε) and the insolation use factor (IUF). The effectiveness describes the heat transfer characteristics of the collector and is shown to be a function of a solar number of transfer units (SNTU) and a parameter ψ. For an air-heating collector, the ψ parameter equals the collector efficiency factor, while for a liquid-heating collector it must account for the difference between the plate and tube heat transfer areas. The effectiveness and SNTU parameters are similar to the effectiveness and NTU parameters used in heat exchanger design methods. The IUF is a measure of the operating conditions of the collector. It represents the difference between the transmittance-absorptance product and the ratio of the minimum heat loss to the insolation on the exterior cover. The relationship between the effectiveness and the SNTU parameter is general for all nonconcentrating collectors. One advantage of this method over the traditional Hottel-Whillier method is that it separates the heat transfer characteristics of the solar collector from its optical properties and the operating conditions.

Thermal performance of air-heating solar collectors with thick, poorly conducting absorber plates

Expressions are derived for the efficiency and loss factors of a coverless, air-heating solar collector in which it is possible to assign finite values for the thickness and thermal conductivity of the absorber plate. In the geometry treated, air flow is beneath a flat absorber, and heat transfer is both steady state and one dimensional. The expressions are validated by outdoor measurements from a fullscale tile roof used as a collector and by indoor measurements from tile and metal roof sections tested in a heating simulator. For the tile roofs, the expressions presented give better agreement with experiment than do expressions based on the usual “thin plate” models since the absorber is thick and poorly conducting. Efficiency characteristics of the full-scale tile roof are also presented. Trapezoidally profiled metal panels are commonly used as a cladding for industrial buildings; it is shown that their performance as collectors may be described by a “fin and tube” model. Information is presented in a format that is useful for design and may be used in computer models of the thermal performance of solar-assisted buildings comprising these, or similar, collection devices.

Comparison of zero testing with conventional testing for flat-plate air-heating solar collectors

We have compared BSE zero testing with ASHRAE outdoor steady-state testing using a verified computer model of a rear-duct air-heating solar collector. The two tests are likely to yield similar results provided that the air inlet temperature is considerably greater than the ambient temperature.

Performance equations for a closed-loop water-heating system using a solar air-heating collector

Performance equations of the HWB type have been derived for a closed-loop water-heating system using a solar air collector. Explicit expressions are given for the collector efficiency and overall heat-loss coefficient. The utility of the performance equations has been established with the help of an experiment in which water is recirculated from a storage tank through the air/water heat exchanger in a closed-loop cycle.

Diurnal efficiency at mid latitudes of solar collectors with various thermal capacitances

A verified, multi-node computer model of a flat-plate, rear-duct, air-heating solar collector is described. Results are presented of the daily averaged thermal efficiencies for a variety of simulated ambient conditions pertinent to mid maritime latitudes, for a range of collectors that differ in their thermal capacitances. The smooth, diurnal cycle of insolation produces a significant spread of thermal efficiencies among the collectors, indicating that simple, approximate models of flat-plate collectors might lead to significant errors in predicting performance in real conditions. Rapid fluctuations in insolation, superimposed on the diurnal cycle, lead to only a slightly greater spread of thermal efficiencies. The effect of thermal capacitance on air outlet temperatures is slight.