A comparison of three different collectors for …

1 SolartechnikPr fungForschungIngenieurschule Rapperswil ITRO berseestr. 10, CH- 8640 RapperswilTel. +41 55 222 46 21, Fax +41 55 210 61 311A comparison of three different collectorsfor process heat applicationsS. Brunold, R. Frey, U. FreiSPF-ITRS olarenergie Pr f- und ForschungsstelleIngenieurschule ITRO berseestr. 10; CH-8640 RapperswilTel.:(+41) 055 / 234 626; Fax: (+41) 055 / 234 400 ABSTRACTIn general vacuum tube collectors are used in solar process heat systems. Another possibility is to use transparent insulated flatplate collectors . A critical point however, is that most of the common transparent insulating materials can not withstand hightemperatures because they consist of plastics. Thus, temperature resistive collector covers combining a high transmisivity with alow U-value are required.

2 One possibility is to use capillaries made of glass instead of results 1 of collector efficiency and incident angle modifier will be presented as well as calculated energy gains forthree different collectors : a vacuum tube collector (Giordano Ind., France), a CPC vacuum tube collector (microthermEnergietechnik, Germany) and a new flat plate collector using glass capillary as transparent insulation (SET, Germany).SolartechnikPr fungForschungIngenieurschule Rapperswil ITRO berseestr. 10, CH- 8640 RapperswilTel. +41 55 222 46 21, Fax +41 55 210 61 3121. INTRODUCTIONIn Switzerland, and similarly in most industrial countries, 25% of the total industrial energy demand is needed for thermalprocess heat applications in the range up to 300 C 2. More than half (57%) of this energy is used to produce heat below 150 the future this lower temperature part of process heat could be enlarged at the expense of the medium temperature range( 150 C - 300 C).

3 For example, many drying applications can be shifted toward a lower temperature level just by extendingdrying time. Doing this, in most cases, even the quality of the dried ware is solar applications to be practical, this shift towards a lower temperature level is very important. Whereas expensive highconcentrating solar systems are needed to generate quantities of steam or oil with temperatures of several hundred degreeCelsius, temperatures below 150 C can be produced by high efficiency standard collectors without or with just lowconcentration of the solar radiation. However, the expression standard has to be considered more detailed:The higher the temperatures needed the more important is the minimisation of the collector U-value.

4 It is in the nature of thethings that this minimisation of the U-value is walking hand in hand with an increasing stagnation temperature. collectors whichcan be used in thermal process heat applications (up to 150 C), reach stagnation temperatures of more than 300 C. Therefore, allelements of the collector , such as absorber-coating, insulation materials etc. have to be able to withstand high a long time vacuum tube collectors were the only choice for this application. As the name suggests, vacuum tubes achievethe required low U-value by evacuating the space remaining between the (hot) absorber and the (cold) collector envelope .Thus, heat conductance of the air is suppressed as well as heat losses due to convection inside the collector . The remaining heatlosses are in the order of 1 W/m2K - 2 W/m2K.

5 They mainly originate in the exchange of thermal radiation between the absorberand the surrounding glass-tube and in piping losses. Since no insulation materials are used to suppress absorber losses noproblems occur with the temperature stability of such flat plate collectors it is a different situation. In addition to thermal radiation losses the air inside the collector , usually atatmospheric pressure, is transferring energy mainly by convection as well as by conduction. Therefore the collector itself has tobe insulated against the surroundings. Whereas the rear of the collector can be easily insulated with a variety of temperature andhumidity resistant opaque materials available on the market, the front of the collector is more problematic since it is exposed tosolar radiation.

6 Transparent insulation materials (TIM) combining high transmittance for solar radiation with low heatconductance are the last years great strides were made in this field. But most of the available TIM s are still not a good choice for highefficiency, high temperature flat plate collectors . Either they cannot withstand high temperatures because they are made ofplastics (most honeycomb and capillary materials) or they are hygroscopic and cannot withstand the humidity inside the collector (aerogels etc.). Thus, efforts were made to realise TIM modules which consist of glass capillaries instead of this paper measurement results of the collector efficiency and the incident angle modifier (IAM) will be presented for anexperimental flat plate collector with a glass capillary transparent insulation.

7 As a comparison , measurement results for a vacuumtube collector and a vacuum tube collector with a compound parabolic concentrator (CPC) are shown. Furthermore, calculatedgross heat outputs based on the measured collector data for these three collectors are fungForschungIngenieurschule Rapperswil ITRO berseestr. 10, CH- 8640 RapperswilTel. +41 55 222 46 21, Fax +41 55 210 61 3132. DESCRIPTION OF THE Test performanceThermal performance tests for the collectors were done according to ISO 3. This Standard contains methods forconducting tests outdoors under natural solar irradiance and for conducting tests indoors under simulated solar irradiance. Themeasurements were made on an outdoor testfacility. As the only difference to the ISO wind speed is in the range of0 to m/s instead of 2 m/s to 4 m/s as the ISO Standard the exception of the flat plate collector two collectors of each type were mounted in series on a solar tracker.

8 Therefore theconditions for the measurements were comparable. In addition to the determination of the efficiency the incident angle modifierwas correct the influence of seasonal deviation on the efficiency two reference collectors (flat plate and vacuum tube collector )are Definition of the reference areaAccording to the ISO Standard the collector efficiency can be based either on the absorber area AA or on the gross area AG: Absorber area : The maximum projected area of an absorber where the solar radiation is admitted. Gross area :The maximum projected area of a complete solar collector module, exclusive of integral means ofmounting and connecting fluid Nomenclaturec0 Algebraical Constant -c1 Algebraical Constant W / (m2K)c2 Algebraical ConstantW / (m2K2)cpSpecific heat capacity of heat transfer fluid J / (kgK)xCharacteristical variable(m2K) / WF' collector efficiency factor-AAAbsorber aream2 AGGross aream2 GKGlobal irradiance in the collector planeW/m2 KIncident angle modifier-TiCollector inlet temperature CTeCollector outlet temperature CTmMean temperature of heat transfer fluid CTaAmbient air temperature CQUseful power extracted from collectorWULO verall heat loss coefficient of collectorW/m2 KVVolume flow ratem3/hVRatio of absorber area to gross area -( )

9 EEffective transmittance-absorption product- Specific density of heat transfer fluidkg/m3 collector efficiency- 0 collector efficiency at x = 0-SolartechnikPr fungForschungIngenieurschule Rapperswil ITRO berseestr. 10, CH- 8640 RapperswilTel. +41 55 222 46 21, Fax +41 55 210 61 Basic equationsThe following equations describe the thermal performance of a solar collector under steady state conditions:Q / AA = F' ( )e GK - F' UL (Tm - Ta)(1)Or expressed in terms of measured parameters:Q / AA = V cp (Te - Ti) / AAThe thermal efficiency is given by: = Q / (AA GK) = F' ( )e - F' UL ((Tm - Ta) / GK) = V cp (Te - Ti) / (AA GK)In reality the heat loss coefficient UL is not a constant but is a function of the temperature of the absorber plate and the ambienttemperature. Therefore we have to obtain the following approach:F' UL = c1 + c2 (Tm - Ta)(2)With the use of (2) equation (1) becomes:Q / AA = F' ( )e GK - c1 (Tm - Ta) - c2 (Tm - Ta)2 Therefore the efficiency results in: = F' ( )e - c1 (Tm - Ta) / GK - c2 (Tm - Ta)2 / GKand with c0 = F' ( )e and x = (Tm - Ta) / GK = c0 - c1 x - c2 GK x2 Determination of the Incident Angle Modifier :The incident angle modifier describes the ratio of the efficiency measured at actual admitted irradiance to vertical = / 0 SolartechnikPr fungForschungIngenieurschule Rapperswil ITRO berseestr.

10 10, CH- 8640 RapperswilTel. +41 55 222 46 21, Fax +41 55 210 61 3153. DESCRIPTION OF THE Vacuum tube collector 4A - evacuated glass tubeB - absorberC - metal headerE - support springD - getterF - copper U-tubeFig. : Vacuum tube collector (CORTEC 2; GiordanoIndustries)The collector presented in this category is a typical Corning collector . In this case it is the CORTEC 2,produced by Jacques Giordano Industries in France. Thereare two modules available, one consists of six vacuumtubes and the other of nine tubes. The module tested at ourinstitute has six tubes. The collectors gross area is m2and its weight is 42 kg. A tube with a length of m and adiameter of m is made of borosilicate glass. Longnarrow flat absorbers, which are made of black-chromecoated copper (manufacturer values: = , = ) aremounted inside the glass-tubes (see fig.)