INTEGRATION OF EVACUATED TUBULAR SOLAR COLLECTORS WITH LITHIUM BROMIDE ABSORPTION COOLING SYSTEMS BY D.S. WARD. G.O.G. LoF

Transcription

1 I, I I INTEGRATION OF EVACUATED TUBULAR SOLAR COLLECTORS WITH LITHIUM BROMIDE ABSORPTION COOLING SYSTEMS '. '. BY D.S. WARD H.S. DUFF J.C. WARD G.O.G. LoF ' t' AUGUST 1978 Solar Energy Applications Laboratory Colorado State University

2 INTEGRATION OF EVACUATED TUBULAR SOLAR COLLECTORS WITH LITHIUM BROMIDE ABSORPTION COOLING SYSTEMS BY D.S. WARD w.s. DUFF.J.C. WARD G.O.G. LoF AUGUST 1978

3 INTEGRATION OF EVACUATED TUBULAR SOLAR COLLECTORS WITH LITHIUM BROMIDE ABSORPTION COOLING SYSTEMS D~S. Ward, W.S. Duff, J.C. Ward and G.O.G. L~f Solar Energy Applications Laboratory Colorado State University Fort Collins, Colorado U.S.A. ABSTRACT By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 percent under conditions of ~T/HT = 75 C m2/kw (~T = collector fluid inlet temperature minus ambient temperature, HT = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70 to 88 C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors integrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III. INTRODUCTION The use of a vacuum such that the pressure is kept below 10-3 torr virtually eliminates conduction and convection heat transfer and limits any heat transport to radiation only [l]. The use of a vacuum between the absorber surface and the cover of a solar collector, therefore, has the advantage of greatly reducing the heat losses from the collector. This is seen by noting that the heat loss rate from an evacuated solar collector (QL) can be written as:

4 2 ( 1 ) Q = e:.. a ( T4 - T4) L s g where a is the Stefan-Boltzman constant, Tis the absolute temperature of the absorber surface (Ts) and glass cover (Tg)' and e:.. is the effective emissivity of the absorber. e:.. depends on the geometry of the absorber; for a cylindrical absorber within an only slightly larger concentric glass cylinder, the areas of the two surfaces are approximately equal and can be described by the same area, A. Thus from [2] we obtain: ( 2) A e:... = ~ l/e:s + l/e:g - l where e: is the emi ss i vi ty of the absorber surface ( e:s) and glass surface (e:g). Using equations (1) and (2), we obtain the heat loss coefficient (UL) of the evacuated solar collector. Calculated values are typically UL~ e:s(8.44 W/m2 C). For absorber emittances of e:s = 0.07 tu J.10, UL varies from 0.6 to 0.8 W/m2 C. Because of the support structure of the absorber surface within the evacuated glass tube and the associated conduction losses, experimental values of UL are typically 2.10 to 2.40 W/m 2 c. 0 Transmissivity-absorptivity products ( a) for tubular glass collectors are equivalent to two glass covers with normal inc idence, i.e., (1a) = 0.80, but the use of anti-reflect ion coati ngs and low absorptance glass allows for values of (1a) of 0.85 to Values of FR' the collector heat removal factor, va1~ considerably wi th the design of the evacuated tube solar collector and the heat transfer fluid utilized (see below); but for liquid-heating collectors are generally high. In general, the combination of very low UL a!"ld high FR(ta) values allow for exceptionally high daily solar collector efficiencies. This is due to the fact that the evacuated tubular collector is able to collect useful solar energy under conditions of limited solar availability. For example,

5 3 the "Solar Threshold" (the minimum solar radiation required for the useful collection of solar energy) for a typical flat-plate collector operating at a temperature 50 C above ambient is 0.25 to 0.30 Kw/m 2. For the evacuated tubular collectors, this same solar threshold is only 0.13 to 0.15 Kw/m 2. The importance of this lower solar threshold is that the solar collector can collect useful energy earlier in the mornings and later in the evenings and under less favorable solar conditions (thus extending each day's operating period, as well as extending the operation of the collector to more days per year). This latter aspect provides for very substantial improvements in average daily collector efficiencies in climatic regions of marginal solar availability. The increase in the solar operating period enjoyed by the evacuated tubular solar collectors also improves the overall performance of a solar cooling system, which requires high temperatures for operating absorption cooling machines. State-of-the-art lithium bromide absorption units typically require generator input temperatures of 70 to 88 C, while ammonia absorption units require temperatures of 90 to 180 C, depending upon the type of heat rejection equipment. The integration of evacuated tubular collectors with absorption cooling then allows for high efficiencies and higher percentage of cooling by solar per unit area of solar collector installed. SOLAR COOLING SYSTEM DESIGNS CSU Solar House I (SHI) can operate in either of two collector/thennal storage modes. The Corning Evacuated Tubular Collector (CETC) mode, with the collector mounted on a test bed immediately south of SHI, has been used predominantly since December of 1976, to provide the solar energy input for operating the building's space heating and cooling system as well as heating domestic hot water. During this period the energy from the roof-mounted

6 4 flat-plate collector system (the original equipment of SHI) has been rejected in a controlled manner by means of an exterior liquid-to-air heat exchanger. A minicomputer and microprocessor were used to control the amount and timing of the heat rejected, in order to allow the flat-plate system to react as though it were in a normal operating mode with the solar house. Space cooling of SHI is accomplished by an Arkla Solaire 3-ton (10.55 Kw) lithium bromide absorption chiller with two 1140 liter cool water storage tanks shown schematically in Figure 1 (operating modes are also indicated). The chiller provides solar generated chilled water to the duct coil whenever there is a space cooling load and the temperature of the hot thermal storage unit is greater than 68 C. Initial use of the thennal storage unit in a particular chiller operating cycle requires that the hot thermal storage be be 80 C or greater. temperature. The auxiliary is used until the tank reaches this If there is no space cooling load, the chilled water is directed to the 11 cool 11 storage (drawing the water out of the 11 warm 11 storage) 11 until the tank is full. Cool 11 storage is then used for space cooling whenever the hot thermal storage temperature is less than 68 C. The auxiliary boiler provides hot water at 82 C whenever chilled water is not available in cool storage and the thermal storage unit temperature is less than 80 C. auxiliary is never used to fi l l cool storage. figure 2 shows the prototype SHI Corning Evacuated Tubular solar collector. Each tube is a m 2 absorber surface, which uses a black chrome selective surface having a longwave emittance of 0.05 to 0.07 and a shortwave absorptance of about The The solar collector array consists of a total of 216 close-packed tubes on the test bed, organized into six-tube modules and and separately manifolded as three distinct subarrays. The total array comprises an absorber area of approximately 40 m 2, occupying a support area of 80 m 2. The tubes in each module are plumbed in series with the modules to each manifold connected in parallel. The solar collector loop contains

7 5 about 190 liters of 50/50 ethylene glycol/water mixture. About one-third of the liquid is in the collectors and collector manifolds. CSU Solar House III (SH III) utilizes the Owens-Illinois evacuated tubular solar collector, the Yazaki Lithium Bromide Absorption Chiller (WFC-6003), and the integrated solar heating and cooling system design shown schematically in Figure 3. Two cool storage tanks are utilized, in a different configuration from SHI, to operate the absorption chiller. As the chiller provides chilled water, 11 Cool Storage is filled and 11 Cool Storage 1 11 is drained, thus the output water temperature from the chiller can be kept as low as possible and not mixed with the warmer water in 11 Cool Storage 1 11 The cool storage tanks provide a temperature difference of about 5 C (9 F) which is sufficient to chill the recirculated room air. In addition to improving the operating characteristics of the absorption chiller, the use of cool storage in SH III permits a smaller capacity chiller. Instead of requiring the air conditioner to be large enough to provide the required cooling under a heavy load condition, the cool storage can be used together with the chiller to provide a larger cooling capacity than the chiller alone. Also, with a smaller chiller capacity and storage, the unit operates nearly continuously which results in maximum operating efficiency. Where normally a 3-ton chiller is required, a 2-ton chiller can be used i nstead (1 ton= 3.5 Kw). This is better than a 33 percent reduction in chiller capacity. For more details, consult reference [3]. A cross-section schematic view of the Owens-Illinois evacuated tube solar collector is shown in Figure 4. Water flows through the feeder tube into the end of the absorber tube and through the absorber tube back to the manifold. The solar collector array on SH III consists of 16 modules of evacuated tube collectors arranged in two rows and each module contains 24 evacuated tubes. Each tube has an outer diameter of 5.7 cm (2.25 inch) and length of

8 6 l.15 meters (3.8 feet) and are spaced as shown in Figure 5, so that each collector module covers an area of about 3 m 2 (32 ft 2 ). The effective collection area, however is only 2.55 m 2 (27.4 ft 2 ) with an absorber area of 2 2 l.05m (11.3ft). The flow pattern of the water through the collector module is indicated in Figure 5. Each of the 24 evacuated tube collectors is in series so that water passes through all 24 tubes in sequence before returning to storage. It takes about 10 to 15 minutes for water to travel from the inlet of the module manifold to the outlet. While the tubes in a module are arranged in series, the modules in the array are arranged in parallel. Space between the tubes is important because reflection from the area between and behind the tubes can be directed toward the collector tubes. SYSTEM PERFORMANCE Tables l and 2 illustrate typical daily space cooling performance in SHI for high and low space cooling loads. Note that, because of the thermal capacitance of the building, the space cooling requirement does not materialize until mid-morning, but continues through the late evening. In addition, the chiller control scheme permits the thermal storage tank to reach a temperature of about 80 C (the point where the Arkla capacity is maximum) before solar is used to supply heat to the Arkla generator. Summer performance data for SHI has also indicated an infrequent cycling of cool storage, on the order of once every 3 or 4 days. This is due to the fact that during heavy space cooling demand, the chiller operates continuously in providing chilled water from the Arkla directly to the duct coils. Normally the thermal storage tank temperature drops below 68 C before the space cooling load is met, and thus solar generated chilled water cannot provide any input to cool storage. Under low cooling demand, the complete cooling

9 7 load is met by solar directly, without the use of cool storage. Because of these control situations, cool storage is utilized only under intermediate cooling loads. However, Duff [4] has provided computer simulations showing that even under cool storage charge/discharge cycles given precedence in the control scheme, that little performance improvement is obtained with the use of cool storage. This appears to be due primarily to the improved transient response of the Arkla unit and the infrequent cycling of the unit experienced in the last performance period. DESIGN CONSIDERATIONS The low heat loss coefficient, UL' of evacuated tubular solar collectors extends the operating temperature range significantly. It is therefore possible to observe very high stagnation temperatures (the stagnation temperature is the temperature of the absorber under an equilibrium, no-flow condition of the collector heat transfer fluid). In tests of the 0-I collector, for example, a stagnation temperature of 280 C (540 F) was observed for a solar intensity of 769 W/m 2 (272 Btu/hr ft2). Under even higher solar conditions, temperatures as high as 350 C (662 F) can be expected. Such conditions emphasize the need for careful design of the solar heating and cooling operating and control systems to prevent boiling of the collector fluid, destruction of the control loop, degradation or destruction of the solar collector or its components, and to avoid the safety hazards of high pressure steam discharge. CSU Solar Houses I and III have experienced boiling of the collector fluid (water or water/glycol mixture) during losses of electrical power to the collector pump and during a pump failure. SH III has also experienced boiling during apparently normal operations. During boil-off conditions, many of the 0-I collector tubes have been destroyed (although a set of replacement tubes have had much less breakage), many of the tube manifold connections have developed leaks (which subsequently caused leaks under

10 8 normal operating conditions), and several control instrumentation sensors located in the collector tubes have been destroyed. SHI collectors have held up well under boil-off conditions and no tubes have been lost. The potential for collector fluid boiling is increased by the control time lag inherent in the SH III Owens-Illinois collector. Because of the high pressure drops and low flow rates associated with the 0-I collector [(e.g., the pressure drop per module for water as the collector fluid is related to the mass flow rate by: (3) L'lp = (0.278)(Jfl) 1 83 where L'lp is the pressure drop per module (psi) and Jf1 is the mass flow rate (lb/hr ft 2 )J, it takes about 8 to 15 minutes for the collector liquid entering the first tube of a module until it exits from the last tube of the same module. During this time lag, collector temperature increases as much as 20 C have been observed. During the cooling season when the storage temperature might be 80 C (but still below the minimum operating temperature of an absorption cooling machine), the collector pump, if turned on at the storage temperature plus 2 C, could not prevent boiling of the collector liquid. It has, in fact, been necessary to install a special control instrumentation circuit to activate the collector pump whenever the collector temperature exceeds 75 C regardless of the storage temperature. It might be anticipated that the very low heat loss characteristic of the evacuated tube might prevent freezing of the collector fluid. This is true to some extent, since the evacuated tube may resist freezing for several days even under cold weather conditions and minimal solar input. However, the collector piping and manifolds are not similarly protected and a water/glycol mixture must be used. This mixture must be sufficient to prevent freezing under all reasonably expected winter conditions. The fact that glycol concentrations of 25 percent by volume in water will yield a

11 9 slush condition (which wi l l not expand sufficiently to burst a pipe), such a condition limits the ability of the collector pump to pump through the collector. It is, in fact, possible that pipe freeze conditions in the collector loop (not exposed to the sun) could occur when the solar collector itself is being heated by the sun. a fact already experienced in SH Under these conditions, the collector will boil; III. It is noteworthy that when the liquid in one tube in an evacuated tubular collector modul e boils, the resultant vapor lock stops the collector liquid flow and causes eventual boiling in all of the tubes of that module. The initial design of the 0-I evacuated tubular solar collector module required a white reflective surface (as part of the roof or collector support structure) to be located directly behind the evacuated tubes. (Because of the close-packing of the Corning collector, this was not essential in SHI). A modification to this design is the use of a shaped, specular reflector directly behind and attached to the evacuated tubes. This modification was ' expected to yield 25 percent more energy than a similar module without the specular reflector. This improvement has been observed at CSU Solar House III on clear sunny days. reflectors actually collect less energy. However, on cloudy days the modules equipped with Averaged over a month, the reflectors appear to increase the energy collection by less than 5 percent. It should be pointed out ~hat the 0-I collector must be installed in a north-south orientation (at a selected tilt angle of 20 to 90 degrees). This is due to the inability of the glass absorber tube to withstand the thermal stresses imposed on a horizontal tube when partially filled with a liquid. The Corning collector, on the other hand, is, on the advice of the manufacturer, mounted with the glass tube horizontal in order to prevent air blockage of the copper collector lines in the absorber. Thus, while the 0-I collectors appear to gain little from the use of reflectors, this is not necessarily true for the use of a reflector on a Corning tube, since the

12 10 optical losses on a north-south oriented tube during the course of one day are radically different from those of an east-west oriented tube. Other significant design considerations include installation of evacuated tubular solar collector arrays, ability to easily test for leaks of a collector module, pressure drops across a collector module and the insertion of control sensors in the collector. For example, the installation of the 0-I collector requires the installation of over 300 separate items per module! While many of the pieces can be assembled as a single unit on the ground, over half must be assembled on site (generally a steeply sloping roof). The Corning collector modules, on the other hand, are fully assembled and need only be connected to the manifolds. A significant problem wi th an on-site assembl y of the solar collector modules (in the case of the 0-I collector) is the inability to conduct a leak tes t of the modules prior to their inst?llation. And, if leaks are discovered, the elimination of the leaks may introduce air into the system, causing vapor locks and bo iling of the module. In addition, with the present design of the collector with tubes above and below the manifold, the collector cannot be drained withou t individual removal and replacement of eac~ lower tube. (It should be noted that a recent modification of the 0-I design places 12 tubes above the manifolds in a 4 by 4 foot module, thereby allowing for easy draining of the collector.) Pressure drops in the evacuated tubular solar collectors have generally been exces si ve. As previousl y noted, the pressure drop in the 0- T coll ector has ca used a ti me lag problem in the control instrumentation, in addi t ion to an increase in pumping power. The Corning collector in SHI uses about 800 watts of pumping power in order to match the flow rate of the flat-plate collector. If solar absorption cooling systems are to be compet itive with electrically driven cooling systems with coefficient of performance of 3 or 4, pumpin g power requirements must be held to a lower level. A study of

13 11 the SHI system has shown that lower collector flow rates, and hence lower pumping power requirements, would, in fact, have been a more economical choice. It is also anticipated that further research and development in evacuated tubular collectors will substantially reduce the high pressure drops. The evacuated tubular solar collectors must provide for inserting a sensor for the collector loop control instrumentation. In fact, each individual module should incorporate a simple means of obtaining the absorber or collector fluid temperature (inside the absorber) for purposes of control sensors and as a maintenance tool (check on flow distribution, etc.). The Corning collector has provided this means on two of the modules. The 0-1 collectors, however, do not have this capability in the SH III installation. REFERENCES [l] Speyer, E., "Solar Energy Collection with Evacuated Tubes", ASME Paper No. 64-WA/Sol, Journal of Engineering for Power, [2] Duffie, J.A. and Beckman, W.A., Solar Energy Thermal Processes, Wiley Interscience; New York, [3] Ward, D.S. and Ward, J.C., "Cooling Subsystem Design in CSU Solar House III", Proc.!SES Conference, Winnipeg, Canada, August [4] Duff, W.S. and Leflar, J.A., "Simulation and Design of Evacuated Tubular Solar Residential Air Conditioning Systems and Comparison with Actual Perfonnance 11, Proc.!SES Congress, New Delhi, India, January Research was supported in part by the Solar Heating and Cooling Branch, Office for Conservation and Solar Applications, U.S. Department of Energy, Washington, D.C.

14 Tempering Valve Circulating Pump Solar Thermal Storage Tonk Auxiliary Boiler ARKLA Chiller Cooling Tower 3-way Control Valve Chilled Water Pump /House / Air Duct Heat Exchanger II II WARM Storage Tank II II COOL Storage Tonk Operational Mode Chiller Direct to Load Cool Storage to Load Chi lier to Cool Storage Control Valve (I) A A B Position (2) A B A Figure 1. Solar House I Solar Cooling Subsystem

15 MOLDED PLASTIC MOUNTING WITH MANIFOLD TUBES ABSORBER SUPPORT CLIPS cm i PYREX GLASS TUBE COPPER ABSORBER PLATE COPPER TUBING TUBE CROSS SECTION Figure 2. Evacuated Tube Solar Collector (Corning Glass)

16 BUILDING COOLING _..._..., COOLING COILS SURGE TANK PUMP ELECTRIC AUXILIARY BOILER ABSORPTION CHILLER COOLING TOWER CHECK VALVE SOLAR THERMAL STORAGE - t PUMP - t '-. AUTOMATIC VAL) ( 1200 c;iol) PUMP HEATING COILS BUILDING HEATING PUMP COLD WATER SUPPLY SOLAR PREHEAT TANK - ELECTRIC HOT WATER TANK DOMESTIC HOT WATER SUPPLY Figure 3. Solar House III System Schematic

17 ~-VACUUM PRESSURE FEEDER TUBE FLUID FLOW AREA SUPPLY I ABSORBER TUBE --=-~r-/.r---fluid FLOW AREA RETURN ---~---...(J.---COVERTUBE SELECTIVE COATING Figure 4. Evacuated Tube Solar Collector Schematic (Owens-Illinois)

18 TOP TUBE HOLDER L - END BRACKET FLOW - FEEDER..-::" i.. TUBE t ~~~~~;::::~~~~~~~END BRACKET Figure 5. Evacuated Tube Solar Collector Module (Owens-Illinois)

19 Hour September 4, 1977 Ta HT \ TTs Qu QD QA QC Qcs ( oc) (MJ) (oc) ( oc) (MJ) (MJ) (MJ) (MJ) (MJ) l 20.6 o.o l l o.o l l 7. l l.4 l o.o l l l l ,q ? l l 45. l l l l l l 91. l 27. l l 43. l l l Daily To ta ls l Table l. High Space Cooling Load Solar House I Daily Performance

20 September 24, 1977 Hour Ta HT Tr TTS Qu QD QA QC Qcs (OC) (MJ) (oc) (oc) (MJ) (MJ) (MJ) (MJ) (MJ) l o.o l l l l l l l l l o.o 0.0. l l. l a.a Daily Totals Table 2. Low Space Cooling Load Solar House I Daily Performance

21 NOMENCLATURE A Absorber area Collector heat removal factor Incident solar radiation on a tilted surface Mass flow rate of collector liquid Auxiliary heat to domestic hot water Total cooling (heat rejected) by combined system (solar and auxiliary) Total cooling (heat rejected) by solar Solar heat to domestic hot water Heat loss rate from an evacuated tube solar collector Useful heat collected by solar collector and delivered to thermal storage Ta Tg Tr Ts TTS UL 6p 6T g a Ambient outdoor temperature Absolute temperature of glass cover Conditioned interior space temperature Absolute temperature of absorber surface Temperature of thermal storage Heat loss coefficient of a solar collector Pressure drop in a solar collector per collector module Collector inlet fluid temperature minus ambient temperature Emissivity of the glass cover surface Emissivity of the absorber surface Effective emissivity of the collector absorber Stefan-Boltzman constant Transmissivity of glass cover times absorptivity of absorber surface of collector