Use of heat pumps with solar collectors for domestic space heating in the United Kingdom

The performance of air to air heat pumps is discussed and it is shown that the machines which are commercially available in the UK have higher running costs when used for domestic space heating than gas or coal fires. It is technically feasible to improve the performance of a heat pump by supplying solar energy as a low temperature heat source, but a method developed for evaluating the financial advantage of such a scheme showed it to be unattractive. It is recommended that more effort should be made to improve the COP of air source heat pumps for domestic space heating in the United Kingdom.     

Influence of debonding in ground heat exchangers used with geothermal heat pumps

Debonding in ground heat exchangers used with geothermal heat pumps may occur for a variety of reasons, such as shrinkage of the backfill materials or surrounding formation, improper grouting and thermal mismatch. The effect of thermal contact resistance on the heat conduction due to debonding in ground heat exchangers was investigated using a set of one-dimensional simplified analytical models as well as two-dimensional finite element models. From the cases studied, debonding at the backfill/pipe interface was found to be of greater significance than debonding between grout and surrounding formation.     

Experimental studies on a ground coupled heat pump with solar thermal collectors for space heating

This paper presents experimental studies on a solar-assisted ground coupled heat pump (SAGCHP) system for space heating. The system was installed at the Hebei Academy of Sciences in Shijiazhuang (lat. N38°03′, long. E114°26′), China. Solar collectors are in series connection with the borehole array through plate heat exchangers. Four operation modes of the system were investigated throughout the coldest period in winter (Dec 5th to Dec 27th). The heat pump performance, borehole temperature distributions and solar colleting characteristics of the SAGCHP system are analyzed and compared when the system worked in continuous or intermittent modes with or without solar-assisted heating. The SAGCHP system is proved to perform space heating with high energy efficiency and satisfactory solar fraction, which is a promising substitute for the conventional heating systems. It is also recommended to use the collected solar thermal energy as an alternative source for the heat pump instead of recharging boreholes for heat storage because of the enormous heat capacity of the earth.     

太阳能集热器动态测试方法简介

本文介绍了目前国际上影响较大的几种太阳能集热器的动态测试方法的原理、实验装置、测试步骤,分析了它们在实验条件要求、实验步骤、应用范围、数据处理方法和精度等方面的差别。     

Radiation heat losses in solar collectors with plastic covers

The use of plastic covers instead of glass covers in solar collectors requires a modification of the calculation of radiation heat loss, because plastics are, in contrast to glass, partly transparent for thermal radiation. This article describes a simple method which enables calculation of radiation heat loss in collectors equipped with plastic covers. Comparison of this method with an incorrect method used by some authors shows that the latter leads to large errors in some cases. The results of the correct method indicate that in some situations the glass covers can be replaced by much cheaper plastic covers with a negligible increase in collector heat loss.     

A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.     

Energy delivery of solar thermal collectors in Zimbabwe

The long-term annual thermal energy delivery per unit of collector area of commonly used collector types and configurations, for a range of operating temperatures, are calculated for representative locations in Zimbabwe. A well-known model found in the literature is the basic tool of analysis, the only modifications being, the use of a locally-derived correlation of the monthly average diffuse fraction of hemispherical radiation to the monthly average clearness index, and the use of temperature-dependent collector heat loss coefficients. The results are presented as plots of annual specific thermal energy output against collector receiver temperature gain (T_r−T_a).The results, though founded on a number of simplifying assumptions on some collector parameters, provide a sound basis for the economic evaluation of solar thermal applications in Zimbabwe.     

The heat efficiency of solar ground water-heating collectors

A numerical heat model of a ground solar water-heating collector (GSWC) has been developed and its heat losses have been investigated. The heat losses in the GSWC and in “ordinary” flat collectors are shown to be similar. The model can be used to choose the constructive parameters of the GSWC and to estimate the temperatures in a ground layer under different conditions on the surface (opened and closed surfaces) and at a varying densities of solar radiation and surrounding temperatures.     

A solar cooker using vacuum-tube collectors with integrated heat pipes

A solar cooking system is described which consists of a vacuum-tube collector with integrated long heat pipes directly leading to the oven plate. The cooker was tested during several clear days in Marburg (latitude 51°). The heat-up times were measured under cold- and hot-start conditions. Detailed temperature distributions and their time dependences were measured. The maximum temperature obtained in a pot containing 51 of edible oil was 252°C.     

Design and analysis of a novel low-temperature solar thermal electric system with two-stage collectors and heat storage units

The proposed low-temperature solar thermal electric generation is based on the compound parabolic concentrator (CPC) of small concentration ratio and Organic Rankine Cycle (ORC). The technologies of CPC and ORC are analyzed, and feasibility of the system is demonstrated. In particular, two-stage collectors and heat storage units are adopted to improve heat collection efficiency. Organic fluid is preheated by flat plate collectors (FPCs) prior to entering a higher temperature heat exchanger connected with the CPC. The two-stage heat storage units are composed of two types of phase change material (PCM) with diverse melting temperatures. The novel configuration is carefully designed to react to different operating conditions. The fundamentals are illustrated for both simultaneous and separate processes of heat collection and power conversion. Mathematic models are built for heat transfer and thermodynamics of the innovative system. Coupling relationship among the proportion of FPC to CPC, the melting temperature of the first-stage PCM and the overall collector efficiency is established. The benefits of the preheating concept and cascaded heat storages are investigated in detail in comparison with the single-stage system. The results indicate that the increase in collector efficiency of the two-stage system is appreciable.     

新型热管式真空管太阳能集热器

分析了新型热管式真空管太阳能集热器的开发背景,介绍了其工作原理,对集热元件、集热盒内关键零件的结构和特点以及产品的固定安装、连接方式进行了详细说明,同时提供了产品热性能测试报告和工程应用实例.     

Assessment of three types of heat pipe solar collectors

New comparative tests on three different types of heat pipe solar collectors are presented in this paper. These three collectors are installed in parallel and tested at the same working conditions.Results are also presented in terms of efficiency plotted against temperature of the heat transfer fluid entering the collector minus the ambient air divided by the global solar irradiance upon the aperture plane of the collector. This allows representing the comparative characteristics of the three collectors when operating under variable conditions, especially with wide range of incidence solar radiation.     

Comparison of three different solar collectors integrated with geothermal source for electricity and hydrogen production

In this study, an integrated system is proposed for mainly electricity and hydrogen production. Energy and exergy analyses of the system are also examined by using Engineering Equation Solver (EES, version 2019) under solar radiation during day time on 1st July. The proposed system consists of a middle-temperature geothermal source with fluid temperature 93 °C, three solar collectors (SCs of 300 m²) namely parabolic trough solar collectors (PTSCs), evacuated tube solar collectors (ETSCs), flat plate solar collectors (FPSCs), an organic Rankine cycle (ORC), proton exchange membrane (PEM), a compressor, hot water storage tank and a mushroom cultivation room. The temperature of the geothermal fluid is upgraded via solar collectors by harvesting solar radiation to operate the ORC. Thus the generated electricity is used in the PEM electrolysis system for producing hydrogen. When the PTSCs, ETSCs, and FPSCs are integrated with the geothermal source separately, it is found that 2758.69 g, 1585.27 g, and 634.42 g of hydrogen can be produced, respectively for a day. The highest overall energetic and exergetic performance of the system is calculated as to be 5.67% and 7.49%, respectively.     

Potential of carbon nanohorn-based suspensions for solar thermal collectors

The optical characterization is reported of a new fluid consisting of single-wall carbon nanohorns and ethylene glycol for solar energy applications. Carbon nanohorns play a significant role in enhancing sunlight absorption with respect to the pure base fluid. The obtained results are compared with those obtained for fluids suspending more conventional carbon forms, i.e. carbon-black particles. We found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs.     

抛物槽式集热器热效率研究

介绍槽式集热器的结构及其工作过程,对集热器进行热性能分析,研究已有集热器热力学模型,并对其进行优化,利用该模型计算各个部位的热损失大小以及集热器热效率,分析得出影响集热器热效率的主要因素,定量分析这些因素对集热器效率的影响趋势,并解释其原因。     

热管技术应用于太阳能平板集热器的研究

介绍了将热管技术应用于平板集热器,以克服其易冰冻、管内结垢、腐蚀等常见问题,论述了设计中需关注的影响因素,并针对热管及翅片的结构优化、加工工艺等进行了探讨.     

Experimental performance analysis and optimisation of medium temperature solar thermal collectors with silicon oil as a heat transfer fluid

Heat transfer fluids (HTFs) play an essential role in solar water heating systems by transferring collected energy from the collector, perhaps via a heat exchanger to the store. If the store is at a much higher temperature than the fluid, the store acts as a heat source, whereas the fluid acts as a coolant, thus reversing the collection process. This action must be avoided through good controls. Experimental performance analysis and comparison of three different types of solar collectors; a non‐concentrating evacuated tube heat pipe and two concentrating single‐sided and double‐sided coated evacuated tube heat pipes collectors are installed and tested using Dow‐corning 550® silicon oil as an HTF under the same operating in‐door control conditions, and results are presented in this paper. The performance of these solar collectors was determined from the overall increase in inlet and outlet fluid temperatures, overall fluid temperature differential, energy collection rate, optical efficiencies, and thermal performances. Temperature differential, energy, and collection efficiency diagrams plotted against time were used to represent and compare the solar collectors. Finally, a comparative analysis of these solar collectors using either pressurised water or Dow‐corning 550 silicon oil as HTF is presented. Copyright © 2011 John Wiley & Sons, Ltd.     

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.