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Design Considerations for Residential Solar Heating and Cooling Systems Utilizing Evacuated Tube Solar Collectors

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Design Considerations for Residential Solar Heating and Cooling Systems Utilizing Evacuated Tube Solar Collectors DESIGN CONSIDERATIONS FOR RESIDENTIAL SOLAR HEATING AND COOLING SYSTEMS UTILIZING EVACUATED TUBE SOLAR COLLECTORS Dan S. Ward and John C. Ward Assistant Professor of Civil Professor of Civil Engineering and Physics ~ngineering ·Solar Energy Applications Laboratory Colorado State University Fort Collins, Colorado 80523 ABSTRACT As solar heating systems become a commercial real- auxiliary use}, and 60 to 80 percent auxiliary on ity, greater efforts are now being employed to in- 20 to 30 days during the heating season . [2] In corporate solar cooling components in order to addition, sizing of DHW, space heating, and space obtain a complete solar heating and cooling system, cooling systems will vary considerably. (See, for and thus take advantage of the cost-effectiveness example, reference 3.) of year-round use of the solar equipment. The solar heating and cooling system design presented in this The perfonnance of solar heating ana cooling sys- paper incorporates design considerations which have tems is strongly dependent upon the operating been obtained from previous experimental efforts characteristics of the various subsystems and, in utilizing evacuated tube solar collectors. These particular, upon the characteristics of the cooling advanced collectors are capable of significantly method. At present, most residential solar cooling higher efficiencies, even at the higher tempera- systems utilize absorption cooling units (usually tures required for solar cooling operation. Most lithium-bromide}. Other possibilities include: of the considerations presented here are based on (l} the use of night air cooled to the wet bulb the experience gained in the design and perfonnance temperature to chill a pebble-bed rock storage unit of the so1ar heating and coo1ing systems for CSU for daytime use (generally limited to arid and So1ar Houses I through IV. semi-arid regions}; (2) radiative-evaporative cooling; (3) solar assisted desiccant dehumidifica- tion; (4) Rankine-vapor compression (these are INTRODUCTION generally only available in very large units}; (5) solar assisted heat pumps; (6) solar Rankine The addition of a solar cooling capability to a heat pumps; (7) heat pump cooling of rock ·beds at solar space and domestic hot water (DHW} heating night; and (8) roof water ponds. However, because system al1ows for a substantia1ly improved usage of the corrmercial availability of lithium-bromide of the so1ar collector array and the associated absorption cooling units, these cooling systems components of the solar system. Year-round usage and their operating characteristics will be used gives greater cost-effectiveness of the solar as the basis for discussion of incorporating eva- equipment because of greater cost savings on con- cuated tube solar collectors into solar heating and ventional fuels in return for a re1ative1y small cooling systems. additional initial capital investment. An inte- grated so1ar heating and cooling system (including The particular characteristics of the evacuated DHW heating} is expected to achieve a much lower tube solar collectors also play an important part cost per unit energy than a so1ar space and DHW in overall system design. For example, because of heating system in most areas of the continental the geometrical shape of evacuated glass tube solar United States. [l] There is, however, the possi- collectors, snow removal can be expected to require bility of technical complications which must be longer melting periods than conventional flat-plate resolved before any cost advantages can be realized. solar collectors, because snow slide-off is gener- A simp1e example is the inability (without signifi- ally impeded by the tubular design. On the other cant and costly complications} to provide for an hand, wind and hail will be much less of a threat optimum ti1t of the collector for both surmier and to the structural integrity of an evacuated glass winter solar angles. (Generally, it is necessary tube . to choose either the winter heating or the surrmer cooling for optimum tilt of the collector, because a compromise angle between the two alternatives EVACUATED TUBE SOLAR COLLECTORS would actually optimize the co1lector for spring and fall solar radiation.} In addition, there are Evacuated tube solar collectors pennit the use of a substantially different operating characteristics vacuum of sufficient magnitude to virtually elimin- of the various systems. Solar cooling would ate convection and conduction heat transfer losses. typically provide about 80 percent of the summer In addition, these collectors generally require a cooling load on a daily basis (with auxiliary minimum amount of material per square foot of col- providing 20 percent each day}, whereas heating is lector and thus provide for the possibility of usually 100 percent solar on most days (no lower costs (under conditions of large scale 10-9 ., I manufacturing processes). Fina 11 y, the vacuum may fl at-plate solar collectors) under the same con- help to protect a selective surface used on the ditions of high outlet fluid temperatures (Tf). absorber (for reduction of long-wave radiation heat This is especially important because of the high losses) against perfonnance degradation over the temperature requirements of solar cooling equip- life of the collector. ment . The input temperature to the generator of a Li Br absorption unit should be (for cooling water The perfonnance of t~ese evacuated glass tube solar t empe ratures of 30°C) between BO and 100°C. For collectors can be represented (for steady-state lower cooling water temperatures (24°C), the range conditions) by an equation of the for:m: is 70 to 100°C. Under these conditions, the Co- efficient of Perfonnance (COP) is in the range of TrT n = n - U ( _ _a) (l) 0.65 to 0.70. While even lower generator inlet 0 5 temperatures can be tolerated for absorption units where n is the solar collector efficiency (dimen- equipped with an internal pump, the resulting de- crease in COP makes lower operating temperatures sionless), n0 is the solar collector efficiency when Tf-Ta = 0 (dimensionless), U is a function infeasible. of the various heat transfer coefficients applic- able to a particular collector (kj/[hr][m2][°C]), The temperatures required to effectively operate Tf is the outlet fluid temperature (°C), Ta is the absorption cooling units emphasize the necessity outdoor air temperature (°C), and S is the solar of using high performance co llectors, capable of radiation on the collector (kj/[hr][m2]). high efficiencies even under conditions of high collector fluid temperatures and high temperatures For comparison purposes, value of no and U are in thermal storage. For example, if the minimum given in Table l for several different kinds of realistic input temperature to the generator of the solar collectors. Equation (l) can also be used to absorption machine is B0 °C, the thermal storage estimate temperature that may be reached during temperature must exceed B0°C. If a heat exchanger stagnation conditions (no flow of coolant fluid is located between the collector and storage, then through collector loop). Under these conditions, the collector is operating at minimum temperatures n 0 and Equation (l) reduces to: of B5 to 90°C. For ambient temperatures of 30°C, = and good solar conditions of 900 W/m2, a typical ( 2) flat-plate collector may achieve peak efficiency of only 40 percent, whereas an evacuated tube solar collector could operate at a peak efficiency of 60 percent . For daily efficiencies the evacuated tube It will be noted that the values of n0 (column 5 of Table l) are higher and the values of U (column 6 can be expected to collect twice as much useful of Table l) are lower for the evacuated glass tube energy as the flat-plate collector. This is due , solar collectors compared to the other more conven- in part, to the fact that the minimum intensity of tional (concentration= l) flat-plate solar collec- solar radiation necessary for the collection of tors listed, but their stagnation temperatures are useful heat is lower for an evacuated tube solar also much higher (column 7 of Table l ). For values collectors, and thus for a given system and identi- of (Tf- Ta)/S > 0.033 (hr)(m2)( °C)/kj, both evacua- cal climatic conditions, the evacuated glass tube ted glass tube solar collectors outperform concen- solar collectors will have longer daily operating trating (concentration= 10) solar collectors that intervals (i.e., they begin collecting useful track the sun. energy earlier in the day and continue longer in the afternoon before ceasing operation.) The superior perfonnance of evacuated glass tube The present major disadvantage of evacuated tubular solar collectors provides for substantially higher collectors is the high cost (ranging from 300 to efficiencies (as much as double that of convention~ 500 $/m2). However, the ma terial costs are only Table 1. Typical Values of n0 and U for Several Different Kinds of Collectors Concen- u, Tf-Ta, ere Manufacturer Type of Solar Collector Fluid tration no kj/(hr}(m2)(°C) for n = O* (l) (2) ( 3) ( 4) (5) (6) (7) Fresnel Lens (track mo model Liouid 10 0. B45 6. 16 466 Northrup Fresnel Lens (non-tracking mode) Liquid 4 O.B59 9.67 302 2 Lexan cover plates General Electric (Selective surface\ Liquid l 0.751 13. l 0 194 I) Observed ** 0. 66 l B.3 122 CSU (Solar House Theoretical Liquid l 0 73 12 0 ?O A Solaron (Solar Houses Observed 0.637 13.2 164 II and IV) Theoretical Air l 0.7 14 6 lh1 Selective surface, 2 glass Arretek cover olates Liqui d l 0. 77 12.9 20B Cornino I Solar House Il Evacuated olass tube Linuid l 0.Bl 5.10 540 Owens-Illinois (Solar Hnuse II J) Evacuated glass tube Liquid l 0. 791 3.59 750 *Calculated using equation (2) assuming that S = 3,406 kj/(hr)(m2) = 946 W/m2 **Based on an experimental test module whose sides were not insulated 10-10 'I.
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