BS-6757-1986.pdf

BRITISH STANDARD BS 6757:1986 Methods of test for Thermal performance of solar collectors UDC 697.329.004.15:536.24 Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 This British Standard, having been prepared under the direction of the Refrigeration, Heating and Air Conditioning Standards Committee, was published under the authority of the Board of BSI and comes into effect on 29 August 1986 © BSI 01-2000 The following BSI references relate to the work on this British Standard: Committee reference RHE/25 Draft for comment DD 77 ISBN 0 580 15375 4 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Refrigeration Heating and Air Conditioning Standards Committee (RHE/-) to Technical Committee RHE/25, upon which the following bodies were represented: British Gas Corporation British Precast Concrete Federation Ltd. Chartered Institution of Building Services Engineers Consumer Standards Advisory Committee of BSI Consumers Association Copper Development Association Cranfield Institute of Technology Department of the Environment Department of the Environment (Building Research Establishment) Design Council Engineering Equipment and Materials Users Association Flat Glass Manufacturers Association Institute of Plumbing Institution of Gas Engineers International Solar Energy Society National Centre for Alternative Technology Royal Institute of British Architects Solar Trade Association Swimming Pool and Allied Trades Association Ltd. University College Cardiff The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Aluminium Federation British Plastics Federation Building Services Research and Information Association Copper Cylinder and Boiler Manufacturers Department of Energy (Energy Technology Support Unit) Department of the Environment (Building Research Establishment, Princes Risborough Laboratory) Department of the Environment (Housing and Construction) Heating and Ventilating Contractors Association National House-building Council National Water Council Society of Laundry Engineers and Allied Trades Limited Co-opted members Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 © BSI 01-2000i Contents Page Committees responsibleInside front cover Forewordii 0Introduction1 1Scope1 2Definitions and nomenclature1 3Collector location and mounting4 4Test apparatus5 5Instrumentation and measurement methods9 6Determination of the thermal performance characteristic of a collector under variable solar radiation conditions14 7Determination of the thermal performance characteristic of a collector using a solar simulator16 8Test report19 Appendix A Design of a shade ring20 Appendix B Corrections to measured irradiance at large angles of incidence21 Appendix C Theory of transient test method23 Appendix D Analysis of data for the transient test method27 Appendix E Computer program for analysis of data using the transient test method28 Appendix F Format for the presentation of test results33 Figure 1 Closed loop test system7 Figure 2 Open loop test system8 Figure 3 Shading of absorber plate24 Table 1 Spectral distribution of simulated solar radiation9 Table 2 Measurement items and accuracies10 Table 3 Coefficients a0 to a4 for 4 mm float glass22 Publications referred toInside back cover Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 ii © BSI 01-2000 Foreword This British Standard has been prepared under the direction of the Refrigeration, Heating and Air Conditioning Standards Committee. The methods of test were originally published as DD 77:1982 in order to gain sufficient experience and information on which to base a British Standard. In the event, DD 77 proved to be satisfactory with only very minor technical changes which have been embodied in the text of this British Standard. It is anticipated that work on this subject will continue, including work by ISO/TC 180, Solar energy, of the International Organization for Standardization (ISO), so that further experience in the use of such methods will be gained during the next few years. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 36, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 © BSI 01-20001 0 Introduction This British Standard describes a method of test for solar collectors for use where the solar irradiance varies during the test, i.e. when the collector is subject to transient conditions. This “transient” method of test has been prepared to complement the “steady-state” method already developed in the USA1). The results of the “transient” test method are expressed as a thermal performance characteristic of the same form as that obtained by the “steady-state” method, with which is has been found to agree to within about 3 % for a range of flat plate collectors. The procedures in the “transient” method for both the collection of the test data and more particularly for its subsequent analysis have been developed especially for this British Standard. The theory of the analysis is described in Appendix C. A “steady-state” method of test for use in solar simulators is also described in this British Standard. Details are given for the specification of the simulator and for the correction necessary to take account of the extra thermal irradiance present indoors. The requirements for test facilities, instrumentation and reporting formats used in this British Standard are as far as possible in accordance with the Recommendations for European Solar Collector Test Methods2). 1 Scope This British Standard describes methods of test for the thermal performance of a non-concentrating solar collector using a liquid heat transfer fluid which enters the collector at a single inlet and leaves the collector at a single outlet. It may also be applied to a collector with more than one inlet or outlet, or to a combination of collectors, provided that the external piping is connected so as to provide effectively a single inlet and a single outlet. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions and nomenclature 2.1 Definitions For the purposes of this British Standard the following definitions apply. 2.1.1 absorber plate (absorber) the element of a collector that receives and absorbs solar radiation and converts it into heat 2.1.2 absorptance the ratio of the solar radiation absorbed by a surface to that which falls on the surface 2.1.3 ambient air temperature the temperature of the air surrounding the collector as measured by a thermometer in a vented enclosure and shielded from solar, sky and ground radiation 2.1.4 angle of incidence the angle between direct solar radiation and the normal to the collector aperture 2.1.5 aperture that part of the outer surface of the collector which admits solar radiation to the absorber plate, including any glazing bars or supports over the absorber 2.1.6 aperture area the maximum area projected by the aperture 1) ASHRAE Standard 93-77, “Methods of testing to determine the thermal performance of solar collectors”, The American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc., 345 East 47th Street, New York, N.Y. 10017. 2) Solar collectors: Test methods and design guidelines (1985), W.B. Gillett, J.E. Moon (eds), D. Reidel, Dordrecht, for the Commission of the European Communities. Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 2 © BSI 01-2000 2.1.7 collector (solar collector) the general term for a device in which solar radiation is absorbed and converted to heat which is removed by a heat transfer fluid 2.1.8 collector efficiency factor the ratio of the efficiency of a collector to the value it would have if the whole absorber plate were at the mean fluid temperature 2.1.9 collector flow factor the ratio of the collector heat removal factor to the collector efficiency factor (FR/F½) 2.1.10 collector heat loss coefficient the product of the collector efficiency factor and the overall heat loss coefficient (F½UL), the negative of the slope of the thermal performance characteristic 2.1.11 collector heat removal factor the ratio of the efficiency of a collector to the value it would have if the whole absorber plate were at the fluid inlet temperature 2.1.12 efficiency (of a collector) the ratio of the output power, as heat in the heat transfer fluid, per unit area of collector to the equivalent normal irradiance 2.1.13 emittance the ratio of the thermal radiation from unit area of a surface to the radiation from unit area of a full emitter (black body) at the same temperature 2.1.14 flat plate collector a non-concentrating collector in which the absorber plate is essentially planar 2.1.15 fluid inlet temperature the temperature of the heat transfer fluid as it enters the collector 2.1.16 fluid outlet temperature the temperature of the heat transfer fluid as it leaves the collector 2.1.17 heat transfer fluid the fluid that passes through the collector and removes heat from the absorber plate 2.1.18 irradiance (diffuse solar) the irradiance of solar radiation incident from the whole of the sky except that contributing to the direct solar irradiance 2.1.19 irradiance (direct solar) the irradiance of solar radiation incident from the suns disc and from the circum-solar region of the sky within a subtended half angle of 8° Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 © BSI 01-20003 2.1.20 irradiance (equivalent normal solar) the irradiance normal to the plane of the aperture whose effect on the thermal output of the collector would be the same as that of the total solar irradiance on the aperture 2.1.21 irradiance (thermal) the irradiance of radiation at wavelengths greater than 4 4m 2.1.22 irradiance (total solar) the sum of the direct and diffuse solar irradiance 2.1.23 mean fluid temperature the average temperature of the heat transfer fluid in the collector. Its value is taken to be the arithmetic mean of the fluid inlet and outlet temperatures 2.1.24 non-concentrating collector a collector that does not concentrate onto the absorber the solar radiation passing through the aperture 2.1.25 overall heat loss coefficient the mean heat transfer coefficient from the heat transfer fluid in the collector to the ambient air 2.1.26 reduced temperature the ratio of the difference between the mean fluid temperature in the collector and the ambient temperature to the equivalent normal solar irradiance 2.1.27 thermal performance characteristic a plot of the steady-state efficiency of a collector against reduced temperature 2.1.28 zero-loss collector efficiency the efficiency of the collector when the mean fluid temperature is equal to the ambient air temperature F½(!Ù)0 2.2 Nomenclature For the purposes of this British Standard, the following nomenclature applies. Aaperture area (m2) cspecific heat capacity of heat transfer fluid J/(kg K) Dday of year (dimensionless) Eequation of time (h) F½collector efficiency factor (dimensionless) F¾collector flow factor (dimensionless) FRcollector heat removal factor (dimensionless) Gequivalent normal solar irradiance (W/m2) Gbdirect solar irradiance in plane of collector (W/m2) Gddiffuse solar irradiance in plane of collector (W/m2) GLthermal irradiance in plane of collector (W/m2) GRthermal irradiance typical of outdoor conditions (W/m2) GTtotal solar irradiance in plane of collector (W/m2) GMTGreenwich Mean Time (h) Licensed Copy: sheffieldun sheffieldun, na, Tue Dec 05 06:18:33 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 6757:1986 4 © BSI 01-2000 3 Collector location and mounting 3.1 General. For convenience, the information relating to collector location and mounting for both methods of test specified by this British Standard is gathered together in this clause. Guidance is also given about the choice of test site for the outdoor method of test. The applicability of this clause to each method of test is indicated in clauses 6 and 7. 3.2 Location of collector 3.2.1 Except when using a solar simulator, the collector shall be located where there will be no significant obstructions in its field of view. NOTEThis condition will be satisfied if not more than 5 % of the sky in the field of view of the collector is obstructed. 3.2.2 The collector shall be located such that a shadow will not be cast on to the collector at any time during the test period. mass flowrate (kg/s) Mfluid capacity of collector (kg) output power per unit aperture area (W/m2) T*derived value of reduced temperature for steady outdoor conditions (K m2/W) Taambient air temperature (°C) Tefluid outlet temperature (°C) Tifluid inlet temperature (°C) Tmmean fluid temperature (°C) uair speed (m/s) Ucollector heat loss coefficient W/(m2 K) Ubback loss coefficient W/(m2 K) Ucaheat transfer coefficient from cover to ambient air W/(m2 K) ULoverall heat loss coefficient W/(m2 K) !absorptance (dimensionless) !solar azimuth (degrees measured from N through E) !pcollector azimuth (degrees measured from N through E) (!Ù)deffective absorptance-transmittance product for uniformly diffuse irradiance (dimensionless) (!Ù)Úeffective absorptance-transmittance product for angle of incidence Ú (dimensionless) ¶collector tilt (degrees from horizontal) ¾solar altitude (degrees) $solar declination (degrees N of equator) %tincrement of time over which transient data are averaged (s) it shall be recorded to the accuracy and precision given in Table 2, item a). Time intervals shall be measured, by the same clock or otherwise, to the accuracy and precision given in Table 2, item b). NOTEIt may be convenient to use the data logger as a clock. 5.3 Measurements of solar irradiance 5.3.1 General. The direct and diffuse components of the irradiance in the plane of the collector aperture shall be measured. NOTE 1The desired accuracy of measurement of ± 1 % for these variables is not achievable with the pyranometers presently available. The World Meteorological Organisation has