Comparison of Systems

Transcription

1 Comparison of Systems

2 WORKSHOP IN THE PRACTICAL ASPECTS OF SOLAR SPACE AND DOMESTIC WATER HEATING SYSTEMS FOR RESIDENTIAL BUILDINGS MODULE 10 COMPARISONS OF SYSTEMS SOLAR ENERGY APPLICATIONS LABORATORY COLORADO STATE UNIVERSITY FORT COLLINS, COLORADO NOVEMBER, 1978

3 TABLE OF CONTENTS Page LIST OF FIGURES LIST OF TABLES 10-ii 10-ii INTRODUCTION 10-1 OBJECTIVE 10-1 PERFORMANCE COMPARISONS 10-1 QUALITATIVE COMPARISONS 10-5 ADVANTAGES OF LIQUID SYSTEMS 10-5 DISADVANTAGES OF LIQUID SYSTEMS 10-6 ADVANTAGES OF AIR SYSTEMS 10-8 DISADVANTAGES OF AIR SYSTEMS 10-8 COST OF HEAT DELIVERED 10-9

4 10 ii LIST OF FIGURES Figure Page 10-1 Comparison of Liquid and Air Collectors Based on Measured Performance Results of Performance Calculations LIST OF TABLES Table Page 10-1 Comparison of Typical Solar Heating Systems Employing Liquid and Air Collectors 10-4

5 INTRODUCTION The two major types of systems now available commercially are those which utilize a liquid for transfer of heat from collector to storage and those which utilize air for the same purpose. The so-called passive types, in which collection and storage are combined, are not commercially manufactured because they are so closely associated with the design and construction of the building that they are primarily architectural considerations. OBJECTIVE The objectives in this module are to compare air and liquid solar heating systems quantitatively on the basis of heat delivered from equal areas of similar collectors and qualitatively on the basis of advantages and disadvantages for operation. PERFORMANCE COMPARISONS Efficiency relationships for a typical air collector operating at two different air circulation rates and a typical liquid collector are shown in Figure Whereas flow rate does not significantly affect the efficiency of a liquid collector, it is evident that air flow rate has a significant influence on air collector performance. Although even greater efficiencies can be achieved with higher air flow rates, the larger pressure drop and power requirements to circulate air at rates above 2 cfm/ft force a compromise between collector efficiency and

6 power consumption. As seen from Figure 10-1, the liquid collector is more efficient than the air collector (toward the left-hand side) at the same inlet temperature, ambient temperature, and solar radiation level. It is important to recognize, however, that liquid and air collectors normally operate at very different inlet temperatures. Air collectors usually operate at conditions substantially nearer the left side of the graph than do the liquid type because the inlet temperature can be substantially lower for the air system than for a liquid system. The net result is that comparable operating efficiency can be achieved for the two collectors while operating in their respective systems. Therefore, comparative evaluation of air and liquid systems requires attention to the other components in the system and their effect on collector performance Collector Efficiency Figure Ti- T a H ft-hr- F Btu Comparison of Liquid and Air Collectors Based on Measured Performance (points shown are for air collector operating at 2 cfm/ft 2 )

7 Table 10-1 contains a step-by-step summary comparison of air and liquid collectors in their respective systems. Typical air and water heaters are compared at a high solar radiation level and at a farily low solar intensity. Characteristic designs and operating conditions have been assumed. The results of the two calculations are shown in graphical form in Figure It may be noted that at the high solar radiation level, 300 Btu/(hr ft 2 ), the two collectors have identical (50 percent) efficiency, and at the lower solar level, 150 Btu/(hr ft 2 ), the air collector (operating at the characteristically low return air temperature) has an efficiency substantially greater than the liquid collector. Obviously, other collectors with different efficiency curves will produce different results. The example comparison, however, illustrates that in comparing solar system performance, comparison of collector efficiency curves alone without consideration to system conditions may lead to misleading conclusions. =0.68= F R U L Ti -T 0 Figure Results of Performance Calculations

8 Table Comparison of Typical Solar Heating Systems Employing Liquid and Air Collectors Performance Relationship: Collection Efficiency: Qu = A k F R - F R u C H T L T, - T. Design Characteristics: Heat Recovery Factor F R Heat Loss Coefficient U L Cover Transmission Plate Absorptivity F r F R U L Liquid Air Operating Conditions: Atmospheric Temperature, T, F a Fluid Inlet Temperature, T., F Solar Radiation H, Btu/(hr ft 2 ) Fluid Flow Rate, gpm/ft 2, cfm/ft Calculated Performance: F R U L (T - T A )/H T F r - F R U L (T - T A )/H T Collection Efficiency, % Computed Outlet Temperature, F

9 In terms of system efficiency, or annual heat delivered per unit collector area, the two systems have comparable performance. Several studies have shown that the difference in heat output is small, and that one system may be slightly better under some conditions and the other superior in other situations. The most recent information on two identical adjacent houses shows nearly 20 percent more heat was supplied by the air system from equal collector areas. But a conservative appraisal is that the two systems have approximately equal heat delivery capability per square foot of collector area. More data are needed before more definitive statements can be made. QUALITATIVE COMPARISONS ADVANTAGES OF LIQUID SYSTEMS In comparing air and liquid handling in systems, each has advantages and disadvantages. The primary advantages of the liquid system are due to use of a low-cost fluid with high heat capacity. Relatively small piping for transferring heat from collector to storage and from storage to the heated space in hydronic distribution systems is an economic advantage, particularly in large buildings. The volume of water in which a given quantity of heat can be stored is much less than required of any other material not undergoing a phase change of some type. Heat storage in materials undergoing phase changes is not commercially practical, so water is the most compact heat storage material now available. Another advantage of the liquid system is its capability for solar air conditioning. Although such systems are not fully developed, they do have practical possibilities, particularly in larger industrial and

10 and commercial buildings. An additional advantage in the liquid system is the number of commercial manufacturers of liquid-heating solar collectors. Various styles, materials (aluminum, copper, and steel), transparent coverings (glass, plastic films, and heavy plastics), and sizes are available. Finally, a large amount of experience is available with liquid collectors (originally used for hot-water supply), including theory as well as practice. DISADVANTAGES OF LIQUID SYSTEMS The disadvantages of liquid systems result primarily from the chemical and physical properties of water. Its freezing point, boiling point, and chemical reactivity with metals require designs and materials which can add substantial cost to a solar heating system. In nearly all parts of the United States, water would occasionally freeze in a solar collector and cause extensive damage. A fail-safe drainage system must, therefore, be provided if water is used in the collector, or a nonfreezing liquid must be used, with heat exchange to water storage in a part of the building where freezing cannot occur. A self-draining collector imposes some design restrictions, and the periodic filling of the collector tubes with air imposes limitations on the types of metal which can be used. Nonaqueous heat transfer liquids may be used in the collector loop, but their practical utility has yet to be adequately demonstrated. The corrosiveness of water in contact with aluminum or steel, in the presence of air, is a factor which must be considered in the design and use of water-heating solar collectors. Galvanic corrosion (in the presence of other metals) of aluminum in water must be avoided by

11 suitable non-conducting connections in the system. Pitting corrosion of aluminum in the presence of slight metallic impurities as well as dissolved oxygen and impurities in the water may result in early failure of the aluminum tubes, particularly if thin-walled. Breakdown of antifreeze solutions (ethylene glycol for example) to acidic compounds can accelerate corrosive attack and must be avoided by suitable preventive maintenance. Steel is less subject to attack than aluminum, but precautions must nevertheless be taken. The probable life of a steel collector is greater than that of an aluminum collector having the same tube thickness. Periodic draining and filling with air must, however, be avoided. Copper, at least for tubes, appears to be the most durable and dependable material. The only disadvantage is its substantially higher cost. A plate-type copper collector requires an outlay of roughly three dollars per square foot in excess of that for aluminum. At the retail level, this difference could be as mush as five to six dollars in selling price. With any of the metals used for water-heating collectors, corrosion inhibitors can be added to the solution (whether freeze-protected or not) thereby substantially extending the life of the equipment. The inhibitor itself, however, must be maintained at suitable concentration by periodically checking and adding when necessary. Another disadvantage of the water system is the boiling which occurs if circulation is lost during sunny weather. The system must be designed with appropriate vents or relief valves to permit discharge of steam when these failures occur. If the condition persists for several hours, there can be so much loss of fluid that recharge is then necessary. For typical residential and commercial installations, a maintenance man would have to be called, and additional antifreeze agent (if used), corrosion

12 inhibitor, and water would have to be added. These requirements impose costs which must be considered in any comparison of systems. In a well-designed and maintained liquid system, damage to the building and its contents from liquid leakage should not occur. However, poor maintenance or careless operation can contribute to leakage of the collector fluid or of water from the storage system through one of many joints and connections, or through corrosion sites, and can result in expensive damage. Good preventive maintenance is therefore a primary requirement of satisfactory operation of a liquid system. ADVANTAGES OF AIR SYSTEMS The advantages and disadvantages of an air system are essentially the reverse of those associated with a liquid system. Advantages are the absence of problems associated with corrosion, freezing, boiling, fluid replacement, monitoring of fluid composition, and potential damage by system leakage. DISADVANTAGES OF AIR SYSTEMS A disadvantage of the air system is the larger volume required for heat storage - approximately three times that for the equivalent heat storage capacity in water. This requirement imposes a need for floor space having a linear dimension approximately 60 percent greater than for a cylindrical storage tank. For example, equal heat storage can be provided in an eight-foot cube of pebbles and in a tank of water five feet in diameter and eight feet high. Another air system disadvantage is the size of ductwork between collector and storage. About four square

13 feet needs to be available for two ducts between collector and storage in a typical residential installation. A third disadvantage is the current lack of air conditioning equipment operable with a solar-heated air supply. This situation, however, is not yet a deterrent to air system use because no solar air conditioning system is yet commercial. Comparisons of the advantages and disadvantages of solar heating system types outlined above leads to the conclusion that the air system is superior insofar as durability and freedom from maintenance are concerned. Experience with a limited number of systems bears out this generalization. As to compactness and wide availability of hardware, the liquid system appears to be the better choice. These relative advantages suggest that air systems may predominate in residential installations where maintenance is notoriously neglected, where compactness is often not considered essential, and where durability is important. Liquid systems, on the other hand, may predominate in commercial and industrial installations where maintenance is routinely practiced, where space is frequently at a premium, and where occasional equipment replacement is acceptable if economically desirable. COST OF HEAT DELIVERED The final and conclusive basis for comparison is cost per unit heat delivered. If efficiency, useful life, and maintenance costs are equal, the system requiring the least maintenance per square foot of collector is the best choice. System costs vary widely, and it is difficult to refer a specific system on this basis. However, examination of published prices of solar collectors and consideration of the cost of other components

14 in the system can be made along with installation costs to make an estimate. Another important factor bearing on solar heat cost is the useful life of the system and the costs of maintenance and repairs. On these points there is little doubt that the air system involves lower annual expense. The absence of corrosion, the use of moderate-priced metal (mild steel), and the absence of servicing requirements indicate that the air system will have a longer life and lower maintenance cost than the liquid system. With respect to evacuated tubular collectors, their high efficiency is a great advantage. These units are not yet being made for sale, so it is difficult to make comparisons with flat-plate systems. Manufacturing costs are much higher, and current prices may not reflect true costs. But if these units can be produced in large volume (e.g., a thousand tons of glass per month), costs might reach a competitive level. Selection of evacuated tubular systems today would have to be based on criteria other than cost, such as high temperature delivery of collector fluid at reasonable efficiencies. But when demand reaches the level justifying automated tubular collector production with a furnace used exclusively for this product, costs may become very attractive. In the final choice of a solar heating system, consideration must be given to the type of use which the system is the meet. As previously indicated, liquid systems appear to have some advantages over air systems in large installations where maintenance is customary and where cooling may now or later be provided by solar energy. might also provide incentives for liquid system use. Other circumstances It is evident that both systems have potential for widespread application.