BS 6757:1986 download.Methods of test for Thermal performance of solar collectors.
0 Introduction
BS 6757 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 USA’.
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 BS 6757. 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 BS 6757 are as far as possible in accordance with the Recommendations for European Solar Collector Test
Methods2.
1 Scope
BS 6757 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 ma’ 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.
NOTE The tt1e of the pub1ication referred to in this standard are Iited on the inside back cover.
2 Definitions and nomenclature
2.1 Definitions
For the purposes of BS 6757 the following definitions apply.
2.L1
absorber plate (absorber)
the element of a collector that receives and absorbs solar radiation and converts it into heat
2.1.2
absorutanee
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/F)
2.1.10
collector heat loss coefficient
the product of the collector efficiency factor and the overall heat loss coefficient (PU1), 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
3.2.3 The collector shall be located where there will be no significant solar energy reflected on to the collector from surrounding buildings or any other surfaces in the vicinity during the test period.
NOTE The reflectance of most rough surfaces, such as grass, weathered concrete or chippings is usually low enough to satisfy this condition. Surfaces to be avoided in the collector’s field of view are large expanses of glass, metal or water.
3.2.4 The collector shall he located so as to allow the free passage of air in front of and behind the collector.
3.2.5 The collector shall be located such that within its field of view there will be no significant sources of thermal radiation at temperatures different from ambient.
NOTE If care is taken inchoosmg a site devoid of adjacent surfaces that could be significantly warmer than ambient, this condition will be met. For example, it is essential that care is taken when locating the collector on the roof of a building to ensure that there are no chimneys (including possible exhaust plumes) in the collector’s field of view, In a solar simulator the area of surfaces in the field of view of the collector which are not at ambient. temperature should be minimized. It is important that the thermal irradiance at the collector aperture does not exceed that of a black body at ambient temperature by more than 100 W/m2 (see 4.7.6)
3.3 Mounting of collector
3.3.1 Collectors not designed as free standing units, collectors not designed as free standing units (e.g. those which do not have a weatherproof casing, or are designed to be an integral part of a roof structure) shall be mounted in accordance with the manufacturer’s recommendations and with those conditions of 3.3.2 which are not in conflict with the manufacturer’s recommendations. Where the conditions are not met this shall be reported with the test results (see F.2).
3.3.2 Collectors designed as free standing units. The collector shall be mounted such that its lowest edge is more than 0.5 m above the local ground or floor surface.
The collector mounting shall not obstruct the aperture of the collector.
Except when using a solar simulator, the collector shall either be mounted in a fixed position facing approximately south or be moved to follow the sun in azimuth. If the angle of incidence is not recorded directly, the collector azimuth shall be determined to within ± 2° for each collector position.
The angle of tilt shall be determined to within ± 2° and stated in the test report (see F.2).
NOTE The collector should normally be mounted such that the angle of tilt between the plant’ of the aperture and the horizontal can be fixed at 45 ± 100. but if it is necessary to perform the rest at some other angle of lilt. e.g. one corresponding to a particular roof structure, then this is permitted provided that the angle of tilt is clearly stated in the test report (see F.2).
4 Test apparatus
4.1 (;eneral
4.1.1 This clause describes the test apparatus, other than the location and mounting of the collector, for the methods of test described in clauses 6 and 7.
4.1.2 The purpose of the apparatus is to provide a steady flow of fluid through the collector, in such a manner that the fluid inlet temperature can be held at any value between ambient temperature and the maximum fluid inlet temperature adopted for any test in accordance with 6.5.1 or 7.5.1.
7.7.2 To determine the outdoor thermal performance characteristic the following assumptions shall be made where separate measurements are not carried out:
a) € is given the value 0.9:
b) Ub is given the value I W/(m2 K);
c) P is given the value 1;
d) U is given the value 25 WI(m2 K);
e) 0R is given the value — 50 W/m2.
Using the mean values recorded in F.5.2 for the variables q. Tm — T, and G and the value of G1, recorded in F.4 b), values of q and 7” shall be obtained and recorded in the format given in F.7. Estimates of the values of the parameters q, and U shall then be obtained as the intercept and slope of the best straight line fit obtained using the method of least squares to a plot of the values of q against 7”.
8 Test report
The results of the tests shall be presented in the format given in Appendix F.