CSU SOLAR HOUSE III SOLAR HEATING AND COOLING SYSTEM PERFORMANCE

Transcription

1 C Distri bution Category UC-59c CSU SOLAR HOUSE III SOLAR HEATING AND COOLING SYSTEM PERFORMANCE ANNUAL REPORT - EXECUTIVE SUMMARY FOR THE PERIOD 1 OCTOBER 1978 TO 30 SEPTEMBER 1979 Dan S. Ward John C. Ward H. S. Oberoi Solar Energy Applications Laboratory Colorado State University Fort Collins, Colorado Prepared for the U.S. Department of Energy Conservation and Solar Applications Branch Under Contract DE-AC02-79-CS October, 1980 Solar Energy Applications Laboratory Colorado State University

2 C Distribution Category UC-59c CSU SOLAR HOUSE III SOLAR HEATING AND COOLING SYSTEM PERFORMANCE.. ANNUAL REPORT - EXECUIIVE SUMMARY FOR THE PERIOD 1 OCTOBER 1978 TO 30 SEPTEMBER 1979 Dan S. Ward John C. Ward H. S. Oberoi Solar Energy Applications Laboratory Colorado State University Fort Collins, Colorado Prepared for the U.S. Department of Energy Conservation and Solar Applications Branch Under Contract DE-AC02-79-CS October, 1980

3 NOTICE This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any.legal liability or responsibility for any.third party ' s use or the results of such use of any information, apparatus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned r ight. This work has been supported by the Solar Heating and Cooling Research and Development Branch, Office of Conservation and Solar Applications, U.S. Department of Energy Printed in the Unit~d States of Ame r ica Available from National Technical Information Center U.S. Department of Commerce 5285 Port Royal Road Springfield, Virginia 22161

4 TABLE OF CONTENTS INTRODUCTION RESULTS General System Efflciency So 1 ar Operating Thre.sb.o 1 d Thermal Storage Heat Losses Piping Heat Losses. Electrical Power Requirements. Cool Storage Feasibility. Solar Collector Heat Capacity. System Versus Collector Efficiency. SYSTEM CONFIGURATIONS PERFORMANCE IMPROVEMENTS. CONCLUSIONS LIST OF SYMBOLS l

5 EXECUTIVE SUMMARY INTRODUCTION During the period 1 October 1978 through 31 March 1979, the Chamberlain liquid-heating state-of-the-art flat-plate solar collector was evaluated for a complete heating season. The data acquisition and analysis were a continuation of the cooling season performance evaluation conducted during the summer and fall of RESULTS Significant and important results of the systems evaluation included: General The demonstration of the critical importance of temperature differentials between the collector outlet and the absorption chiller generation inlet, the effects of alternative control strategies, the marginal feasibility of cool storage, the devastating effect on system performance due to heat losses from the thermal storage unit, and the importance of minimizing electrical parasitic power requirements in bbtaining feasibility for solar absorption cooling systems. System Efficiency The concept of overall system efficiency was shown to be useful in the determination of the feasibility of any solar installation. The overall system efficiency is defined as: where Qs ns = -Al-+-E/_n_E ( 1) (2) (All parameters are defined in the section on List of Symbols). In addition, a solar conventional energy coefficient of performance, S, defined by equation (3) is of importance. CQunE s = --=-- Enc (3) u(tm+tit 2 +tit-t) I = o Tel (4)

6 2 Analysis of the data from CSU Solar House III indicates that good solar system effi.ciencies. and high values are realizable and that solar can be competitive with conventfonal heating and cooling systems. Solar Operating Threshold Evaluation of acquired data on the performance of the CSU Solar House lii system has provided the theoretical construction and experimental verification of the importance of solar operating threshold (i.e., the minimum solar radiation intensity at which the collector can collect useful solar heat). In addition, the effects of collector loop heat losses, variations in control strategies, and parasitic electrical power requirements on the operating threshold have been clearly quantified and their importance in the design process justified. The House III data indicate the need for a redefinition of the solar operating threshold, particularly in computer simulations. We recommend equation (4). Because of this redefinition, control system variations can be shown to be extremely important in the optimal thermal performance of the solar heating and cooling system. Simple control logic and reliable control instrumentation and sensors are of critical importance. Thermal Storage Heat Losses The importance of thermal storage heat losses to the interior of the heat space has been shown to be a critical factor in determining the feasibility of active solar cooling systems.. The losses from the thermal storage tank were found to represent a significant portion of the cooling load and were of such magnitude as to threaten the technical feasibility of solar absorption cooling. In addition, the low heat capacity of the auxiliary electric furnace led to a substantial degradation in the COP of the chiller when operating on auxiliary. The COP of the chiller was found to be about 25% less when operating on auxiliary as compared to solar. Because of this and related factors, it is suggested that auxiliary cooling should

7 3 ' utilize conventional cooling machines in lieu of using auxiliary fuel to operate an absorption cooling unit, particularly for residential applications. CSU Solar House III cooling data, acquired during the 1978 summer, have provided substantial evidence that shows the non-feasil:>fltty of solar absorption (LiBr} cooling systems whenever conventional thermal storage units (utilizing water or other similar liquids) are located inside the conditioned space of the building. The importance of thermal storage heat losses cannot be overemphasized in connection with solar cooling systems. Piping Heat Losses The demonstrated effect of heat losses from system piping and thermal storage units, control strategy variations, heat capacities in system piping, and parasitic power requirements on the solar system 1 s efficiency has been detailed. In addition, it has been clearly shown that neglect of these critical factors in the design process can easily lead to solar installations which consume more energy than can be usefully acquired by the solar system. Electrical Power Requirements Experimental evidence and subsequent analysis have provided a clear basis for the evaluation of solar heating and cooling systems by relating the useful heating and/or cooling by solar to the parasitic electrical power requirements of the solar system. The absolutely essential necessity of minimizing electrical power requirements (to the greatest possible degree) had been clearly delineated. Cool Storage Feasibility The n arginal feasibility of one cool storage subsystem has been shown by reference to interior space requirements and additional (unwanted) parasitic power requirements. However, cool storage may be necessary for some solar chilller units.

8 4 Solar Collector ~eat Capacity The importance. of solar collector hea.t capaci,ty for liqutdheating collectors i'nte.grate.d with a solar heating and cooling system has been demonstrated. (previous researchers have tended to ignore heat capacity effects.} System Ve.rsus Collector Efficiency A test was run under operating conditions to determine the effect of reduced collector flow rate and collector efficiency on the system efficiency. It was found that not only was there a saving in parasitic power by reducing flow rate by half the original value, but there was also an increase in daily collector efficiencies of three percent. This increase in collector efficiency can be explained by the fact that, during the cooling season, the majority of the load occurs during the day. At the lower flow rate the heat collected by storage is directly supplied to the chiller by what constitutes an effective short circuit across the top of the storage unit. This leads to a greater degree of stratification in the tank and therefore the subsequent lower inlet temperature to the collector results in higher daily collector efficiency. Thus cooling performance data indicate that lower collector flow rates (which might be expected to reduce collector efficiency) may actually increase system efficiency by a~lowing for greater temperature stratification in the thermal storage unit. SYSTEM CONFIGURATIONS Experimental data gathered at CSU Solar House III over a period of approximately one year have shown that, for solar heating and cooling to be economically competitive, great care must be taken in designing the system. For a combined heating/cooling and DHW heating system, the design must consider all system energy balances (thermal and electrical). With this goal in mind, the system configuration in CSU Solar House III was redesigned and incorporated the following improvements (see Figure ):

9 MAIN WATER SUPPLY LIQUID TO AIR coourjg -AIR lrllet COIL--- L...~-~--' LIQUID TO -l L l -,...,l..2::~ ~~------i'){i'mi'.'5-ri'c------ir~~~h..., DOMES TIC 1200gol STORAGE TANK AIR HEAT!rJS HOT WATER R-28 POLYURETHANE COIL HEAT EXCHANGER P4 INSULATION ~~... TUBE S SHELL HEAT EXCHANGE1' (t.uxili.c.ry HEATING a COOLJrJG) AIR DUCTJiiG TO HOUSE,----~ RETUR:I AIR 1 1 DUCT FROM L-- - -~ HOUSE CSU Solar House III System Layout

10 6 (1) A significant reduction in parasitic electrical power requirements (2) Piping runs were significantly reduced so associated heat losses were less (3) Thermal storage losses are now primarily to the building exterior and thus do not add to the cooling load (4) The overall system design now more closely resembles a stateof-the-art commercial installation (5) The auxiliary furnace was replaced with a heat pump, which will be utilized to provide either heating or cooling whenever the solar system heating or cooling capacity is insufficient to meet the load PERFORMANCE IMPROVEMENTS The improvement in performance is shown in the Table which compares the important cooling system parameters between the experimental performance and the design predictions of the modified system. Performance Improvements l System Parameter Experimental Modified Actual {per square meter of Performance Design solar collector) (1978) (Predicted) ( 1979) Average daily solar heat delivered to load (month of August, accounting for all heat losses), MJ/day System thermal efficiency 0.05 o Solar system conventional coefficient of performance Average daily non renewable energy savings (MJ/day)

11 7 During the sumner of 1978, Colorado _State University Solar House III uilized a Yazaki 2-ton lithium bromide absorption chiller, two 500 gallon cool storage tanks, one 1200 gallon hot storage tank (located within the conditioned space), 631 square feet of Chamberlain single cover selective surface flat-plate solar collectors, and an electric auxiliary boiler. After a year of heating and cooling data were obtained, the system was redesigned in the spring of Major modifications included reduced piping runs, reduced electrical power requirements elimination of cool storage, improved insulation, and a relocation of the hot storage to the exterior of the conditioned space. CONCLUSIONS The design, operation, and performance analysis of two solar cooling systems has led to several conclusions and recommendations. They are as follows: (1) First, a solar cooling system should be designed to minimize the parasitic power use. This can be accomplished through careful selection and placement of system components to reduce the length and complexity of piping connections. Additionally, appropdately sized, high efficiency pumps and/or blowers should be used. (2) Careful :cnsideration must be given to heat losses and their iocc.tior.. An externally located thermal storage tank and R-30 fos 1.1lat i0n on the tank is extremely desirable and perhaps ~ ss 2ntial for a successful system. Other less obvious means of heat loss, such as conduction through the tqnk support structure and thermosyphoning in load as well as collecto~ loops, should be identified and eliminated if possible. (3) Solar cooling feasibility depends on the specific design and installation variables, such as parasitic power consumption, thermal losses, domestic hot water use, control strategies, building configuration, and many other factors. Therefore an accurate estimation of the performance of a solar cooling system must necessarily include a careful calculation/consideration of these variables.

12 8 LIST Of SYMBOLS Symbol Dafninti.on A solar collector absorber plate a.rea coeffi:cient of performance, COP coefftcient of performa.nce of a conventional mechantcal vapor compresston unit defined by equation 7 parastttc electrtcal energy consumed by tne solar system (electrical energy required to operate a solar neati'ng or!cooli'ng system} I solar radiation tntenstty on the (tilted} solar collector surface tit minimum solar radiation intensity that is co 11ectib1 e solar energy collected by the solar collector solar system heat losses to the exterior of the conditioned space solar system heat losses to the interior of the conditioned space energy saved by using solar energy for heating or cooling solar heat delivered to the cooling load during the surraner ambient air (outdoor) temperature minimum fluid temperature that is useful overall solar collector heat loss coefficient increase in solar collector fluid temperature as it passes through the solar collector ne for electric resistance heating nf for a furnace CcnE for a conventional mechanical vapor compression unit efficiency of electric power generation, transmission, and distribution furnace efficiency overall system efficiency product of the solar collector cover transmittance and the solar collector absorber plate absorptance accounting for dirt, shading, angle of incidence, etc. Units m2 dirnens ion 1 ess day (hr)(m 2 ) day day day day day oc oc (hr}( C )(m 2 ) oc di mens ion 1 ess

13 9 where and _ 1 N CN - Nljl [1-(l-iji) ] 1jl F U = --1U::. lhcp N = Number of solar collector modules in series (7}