Two different concentrating mirrors have been constructed that resemble parabolic dish reflectors. Both mirrors are made of slightly curved strips of flat, bendable material. The strips of the most simplified mirror have only large-radius circles and straight lines as boundaries. The necessary equations for making the mirrors are given. Also a simple way to make a stiff, lightweight frame and support for the mirror strips has been developed. Models of the mirrors have been built and successfully used for cooking and baking.
An international standard, ISO/DP 9459-4A, 1996 has been proposed to establish a uniform standard of quality for small solar heating systems. In this proposal, system components are tested separately and total system performance is calculated using system simulations based on validated component model parameter values. Another approach is to test the whole system in operation under representative conditions, where the results can be used as a measure of the general system performance. Component testing and system simulation is flexible, but requires an accurate and reliable simulation model. The advantage of system testing is that it is not dependent on simulations and that it shows the actual system performance. Its disadvantage is that it is restricted to the boundary conditions for the test. The heat store is a key component concerning system performance. Thus, this work focuses on the storage system consisting of store, electrical auxiliary heater, internal heat exchangers (solar and load loops) and tempering valve. Four different storage system configurations with a volume of 7501 were tested in an indoor system test using a statistically generated six-day test sequence and a solar collector simulator. A store component test and system simulation was carried out on one of the four configurations, applying the proposed standard for stores, ISO/DP 9459-4A, 1996 and the MULTIPORT store model. Three test sequences for internal load side heat exchangers, not in the proposed ISO standard, were also carried out. This paper discusses the results of the indoor system test, the store component test, the validation of the store model parameter values and the system simulations. (C) 1998 Elsevier Science Ltd. All rights reserved.
A new method for the calculation of a stratification efficiency of thermal energy storages based on the second law of thermodynamics is presented. The biasing influence of heat losses is studied theoretically and experimentally. Theoretically, it does not make a difference if the stratification efficiency is calculated based on entropy balances or based on exergy balances. In practice, however, exergy balances are less affected by measurement uncertainties, whereas entropy balances can not be recommended if measurement uncertainties are not corrected in a way that the energy balance of the storage process is in agreement with the first law of thermodynamics. A comparison of the stratification efficiencies obtained from experimental results of charging, standby, and discharging processes gives meaningful insights into the different mixing behaviors of a storage tank that is charged and discharged directly, and a tank-in-tank system whose outer tank is charged and the inner tank is discharged thereafter. The new method has a great potential for the comparison of the stratification efficiencies of thermal energy storages and storage components such as stratifying devices.
Solar combisystems are relatively complex systems with many different components and operational parameters. Before the beginning of IEA-SHC Task 26 ("solar combisystems"), no method was available with which they could be compared. The well known "f-chart" method was introduced by Duffie and Beckman already in the seventies, but was only useful for dimensioning generic combisystems, with a defined hydraulic scheme. It didn't give a method to compare different designs. The objective of this work was to develop a simple tool for characterizing the performance of these systems. The method used was to analyse the comprehensive simulation results of Task 26 and to look for relationships between the key external factors of climate and load, and the system performance. The result is a new and simple methodology for characterization of solar combisystems, called the fractional solar consumption (FSC) method. FSC is a dimensionless quantity, which takes simultaneously into account the climate, the space heating and domestic hot water loads, the collector size, its orientation and tilt angle, but which does not depend on the studied system design. The study shows that fractional energy savings, with and without parasitic energy included, can be expressed as a quadratic function of FSC. The relationship was shown to be valid for a wide range of conditions, but to be limited for certain parameters such as collector orientation and hot water load. The method has been used to create a nomogram and the computer design tool CombiSun.
A new relation between the maximum geometric concentration factor C and the angular acceptance interval for asymmetrical ideal non-imaging concentrators is proposed. A generalization of the well-known relation for the two-dimensional case, sin ?c = 1/C where ?c is the acceptance half-angle, results in the proposed relation sin ?2 -sin ?1 = 2/C , where ?1 and ?2 are the angles of the acceptance interval limits relative to the normal of the entrance aperture. The proposed relation is valid for any ideal 2D concentrator, symmetrical or asymmetrical. A proof based on conservation of phase space is provided.
Solar heating technologies hold a significant potential to supplement or replace the fossil fuel-driven heating systems in residential and industrial applications. This paper presents a techno-economic study aiming to assess the use of Unglazed Transpired Solar Collectors (UTSC) coupled with an energy system assisted by Exhaust Air Heat Pump (EAHP) in cold climates applied to a residential building cluster. The performance of the system and its components is assessed for different sizes of solar collector field. In addition, a rule-based algorithm is developed to manage the airflow into the UTSC, and a comparative analysis is carried out with conventional flow control. The existing EAHP assisted energy system of a multifamily building cluster in Sweden is modelled by using a simulation software TRNSYS, and the effects of the UTSCs integration on the performances of the energy system are evaluated. Results show that the integration of UTSCs has a small but positive impact on the overall system performance. Moreover, the developed control based on the variation of the collector airflow rate for UTSC is an effective control strategy to increase the seasonal performance factor of the overall system and to maximize the savings.
This work presents a discrete model for the thermo-hydraulic analysis of a novel polymeric solar thermal absorber with tapered headers and lens-shaped absorber strings. The numerical model developed is based on empirical correlations for laminar and turbulent flow regimes. The principal aim of this study is to ascertain if tapered headers help improve flow distribution in parallel-flow absorbers under laminar and turbulent flow conditions. In order to verify the model, the simulation results are compared to the measurement for total pressure drop and two studies from the previous literature for flow distribution. Altogether, a good agreement is found in all comparison cases, and the proposed numerical algorithm is proven to be robust and stable for complex thermo-hydraulic analysis. The results are then elaborated by comparing the tapered case to several conventional cases with cylindrical headers but with identical riser configurations to the original model. The results indicate that using tapered headers in compact absorbers with relatively large area ratios can noticeably reduce non-uniformity, especially up to middle range flows, by maintaining higher Reynolds numbers in the headers. The developed model can be used to optimize the hydraulic design of solar collectors with complex geometries and large area ratios. © 2022 The Authors
This article describes movements and mechanical stresses in sealed, gas-filled, flat plate solar collectors. The advantage of a sealed space between a collector absorber and glass cover is that it (i) eliminates the influence of humidity condensation and dust and (ii) it lowers the heat losses when the enclosed space is filled with a suitable gas at normal pressure. However as the solar collector temperature varies, volume and pressure changes cause movement and mechanical stresses. In this study, the finite element method was used to determine movements and mechanical stresses. Several geometries were analyzed and it was found that the stresses in the investigated collectors are a factor 2–4 below the critical stress levels. Furthermore, it was found to be possible to reduce the stresses and improve the factor of safety by (i) using a larger area and/or reducing the distance between the glass and the absorber and/or (ii) changing the length and width so the tubes are longer.
The study includes calculations for both the thermal performance and the mechanical behaviour of a gas-filled, flat plate solar collector without external gas expansion, i.e., a collector with varying gas volume and gas pressure and movement in both cover glass and absorber. Classical theories for the thermal performance are combined with a finite-element method to investigate which factors have an impact from the mechanical stress point of view. This article describes major results for collectors with copper and aluminium absorbers combined with different inert gases. It is shown that a collector may be designed which uses less material than a standard collector but achieves at least the same thermal performance, by using a thinner collector and a thinner absorber and a suitable gas filling other than air. If copper is used in absorber and tubes, a 0.15 mm thick absorber together with a tube-to-tube distance of 103 mm results in the same performance as a 0.3 mm absorber with a 144 mm tube-to-tube distance, but the former will use 25% less material. The use of copper can be further reduced if the absorber is made of aluminium and the tubes are made of copper. The factor of safety for thick (>0.5 mm) aluminium absorbers is, however, not as large as it is for copper absorbers.
A sealed space between absorber and cover glass in a flat plate solar collector makes it possible to reduce the influence of humidity condensate and dust at the same time as the enclosed space can be filled with a suitable gas for lowering the heat losses. This article describes the influence of different gases on the heat losses in a typical flat plate solar collector. A model of a gas-filled flat plate solar collector was built in Matlab with standard heat transfer formulas. The results show that the overall heat loss can be reduced by up to 20% when changing from air to an inert gas. It is further possible to reduce the distance between absorber and cover in order to reduce the mechanical stresses in the material with similar heat losses.
In this paper, a novel solar photovoltaic/loop-heat-pipe (PV/LHP) module-based heat pump system was designed and fabricated for both electricity and hot water generation. A coated aluminium-alloy (Al-alloy) sheet was applied as the baseboard of PV cells for enhanced heat dissipation to the surroundings, which was characterised by a series of laboratory-controlled conditions over the conventional Tedlar–Polyester–Tedlar (TPT) baseboard. The whole prototype system was subsequently evaluated in outdoor weather conditions throughout a consecutive period for about one week. Impact of several external parameters to the PV panel with different baseboards was discussed and the results showed that weaker incident radiation, lower air temperature, higher wind speed, and ground mounting solution, were propitious to the PV electrical performance. Given the specific indoor testing conditions, temperature of the Al-alloy based PV cells was observed at about 62.4 °C, which was 5.2 °C lower than that of the TPT based PV cells, and its corresponding PV efficiency was about 9.18%, nearly 0.26% higher than the TPT based type. During the outdoor testing, the mean daily electrical, thermal and overall energetic and exergetic efficiencies of the PV/LHP module were measured at 9.13%, 39.25%, 48.37% and 15.02% respectively. The basic-thermal system performance coefficient (COPth) was found at 5.51 and the advanced system performance coefficient (COPPV/T) was nearly 8.71. A simple comparison was also conducted between the PV/LHP based heat-pump system and those conventional solar/air energy systems, which indicated that this advanced system harvests larger amount of solar energy and therefore enables enhanced solar efficiency and system performance. Basic analysis into the economic and environmental benefits of this prototype system further demonstrated such technology will be competitive in the future energy supply industry with a payback period of 16 (9) years and a life-cycle carbon reduction of 12.06 (2.94) tons in Shanghai (London).