BUILDING FOR THE FUTURE

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1 BUILDING FOR THE FUTURE The following article was published in ASHRAE Journal, September 4. Copyright 4 American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. Selecting Right System, Configuration By William Ryan, Ph.D., P.E., Member ASHRAE C ogeneration systems generate power and capture heat for local uses. These systems can reduce operating costs, reduce the need for new electric generation, and perhaps, more importantly, reduce the load on electric transmission systems. After last summer s East Coast grid failure, the interest in cogeneration systems is higher than ever. is more acute as there often are few other practical applications for waste heat. Unfortunately, a large body of literature does not exist on the best way to link engine generators and absorption chillers. Surprisingly, even absorption manufacturers offer no specific guidance, although manufacturers sizing programs can be of some help. This is a crosscutting question between HVAC and enginegenerator manufacturers two groups who have had little contact in the past. In the long run, developing countries (where electric demand is growing and electric distribution is strained) can benefit most from cogeneration. Every cogeneration system built reduces the need for central generation and transmission system construction, and decentralizes power production, potentially increasing the security of the electric system. Aggressive year-round heat recovery is important in economically justifying cogeneration systems. Engine generators are the most commonly used drives for cogeneration systems in commercial buildings and campuses. For most practical application sizes, this means that a portion of the summer cooling load must be met by an absorption chiller operating on waste heat from an industrial engine. In the developing world, which tends to be more tropical, the need for cogeneration systems to supply cooling Types The first issue is the selection of the best type of absorption chiller to apply to engine heat rejection. Engine generators reject heat in the exhaust, the jacket water, the oil cooler, one or more turbocharger intercoolers, and directly to the engine room. The last three often are too low in temperature to be practically used. Temperature limits govern how much can be recovered. Engine jacket outlet S30 Building for the Future A Supplement to ASHRAE Journal September 4

2 ABSORPTION CHILLERS Surprisingly, even absorption manufacturers offer no specific guidance, although manufacturers sizing pro- grams can be of some help. temperatures are limited to the 240 F to 250 F (6 C to 2 C) range. Heat recovery mufflers are less limited, but the amount of heat that can be recovered declines with increasing inlet water temperature. Overall, if water at or below 250 F (2 C) can satisfy the load, the jacket water heat as well as a sizable portion of the exhaust heat can be recovered, and heat recovery between 3,800 and 5,000 Btu/kWh (4009 and 5275 kj/kwh) of electric generation is practical. If high pressure (>5 psig [>03 kpa]) steam is needed, the jacket heat cannot be used and more exhaust heat is wasted, lowering heat recovery to as low as,500 Btu/kWh (583 kj/kwh). Given that high-pressure steam is not needed in most commercial buildings, the lower temperature hot water approach can recapture as much as 300% more heat. This means that running a less expensive, single-effect absorber on low-temperature heat is more desirable than using a more efficient hightemperature, double-effect system. Table shows how much cooling is available from such systems. Note that a low-tem- One of two MW generators. perature, single-effect absorber produces more cooling per kw of engine generator at a lower first cost than a high-temperature, double-effect absorber. In addition, single-effect absorbers have somewhat lower maintenance costs than double-effect systems, and do not require steam, eliminating steam system maintenance issues. Lastly, single-effect absorption chillers operate further from the crystallization region than double-effect systems. Figure shows a simple, idealized low-temperature system. Figures 2 and 3 show examples of these components. Engine Integration The way the absorber is connected to the engine is critical for proper operation. Although absorption chillers can be run September 4 at water temperatures as low as 80 F (82 C), operating at such low temperatures may involve a capacity derating. This will require oversizing the absorber, effectively increasing the cost of the absorption chiller in dollars per useable ton. Clearly, operating the energy transfer between the engine and the absorber at the highest temperature practical is desirable. However, the ultimate limitation comes from a source most designers do not initially expect: the maximum temperature of return water to the engine jacket allowed by the engine manufacturer. Industrial engine manufacturers contacted thus far require jacket return water temperature at F (97 C) or below. The return temperature to the exhaust gas heat exchanger or water-cooled silencer is not as limited. So what are the effects on practical supply temperature to the absorber? Figure 5 shows a derating chart for two domestic manufacturers of single effect hot water-driven absorption systems. (Figures 6 and 7 are the same chart with specific sample temperatures.) The multipass line uses data from both manufacturers for chillers with the greatest number of passes available. The chart is somewhat simplistic in that a customer can, by working directly with the manufacturer, order specific changes that can improve capacity somewhat. Therefore, Figure 5 should be used to give a good first estimate of absorber derating. The charts are plotted with inlet temperature on the vertical axis and outlet temperature on the horizontal axis. The capacity factor scale on each line shows the percentage of the original rating these machines produce at any particular inlet and outlet water temperatures. Where the user s system falls on these charts can make a dramatic difference in actual absorber capacity. The chart shows lines for both a single-pass and multipass flow arrangement. In a single-pass arrangement, the hot wa- Building for the Future A Supplement to ASHRAE Journal S3

3 Min. Low Temp. System Max. Min. COP 3,800 Single Effect ,000 Single Effect ,500 Double Effect ,000 Double Effect High Temp. System Max. Percent Cost Above Available, Electric tons/kw (At 500 tons) Gen. Heat Production, Btu/kWh Hot 25% Chilled to 00% Table (left): Comparison of low and high temperature approaches. Figure (right): Ideal engine absorber interconnection. Figure 2 (left): Absorption chiller at GTI cogeneration facility. Figure 3 : heat recovery heat exchanger at GTI facility. ter flows through the absorber generator once before exiting the absorption chiller. In a multipass arrangement, the water flows back and forth through the generator from two to four times before exiting. The longer flow lengths of multipass arrangements remove more heat from each gallon of hot water, resulting in a greater temperature drop through the absorber. The other alternative, running the hot water through the generator tubes more slowly, generally is not practical as the water flow may become laminar and the heat transfer rate may deteriorate. Using these charts for our simplified engine-absorber system, it will be seen that the critical limitation is the return water temperature to the engine jacket. If the maximum return water temperature to the engine jacket is F (97 C), the maximum temperature of water leaving the absorber is F (97 C). This gives the situation shown in Figure 7. Using a multipass arrangement, the absorber could take in water at 230 F (0 C)with a capacity factor of ~84% and still produce the desired F (97 C) outlet water temperature. Feeding water to the absorber at any higher temperature other than 230 F (0 C) would raise the leaving water temperature above F (97 C). This excess heat would have S32 Hot Chilled to F Max. Figure 4: Engine absorber interconnection. to be thrown away before the water reenters the engine jacket, thereby lowering the overall efficiency of the system. With a single-pass machine, the maximum hot water inlet temperature would be F (04 C) and the capacity factor would be ~74%. A multipass arrangement produces significantly less absorber derating than a single pass. In addition, the multipass allows a 23 F (3 C) hot water range (difference between absorber input and output water Building for the Future A Supplement to ASHRAE Journal September 4

4 ABSORPTION HEAT RECOVERY CHILLERS Inlet Temp. ( F) Capacity Factor Single-Pass 40% 00% 00% Inlet Temp. ( F) F F Capacity Factor 40% Single-Pass F 00% 00% Outlet Temp. ( F) Outlet Temp. ( F) Figure 5 (left): Derating charts for two American manufacturers. Figure 6 (right): Operating points to achieve F (97 C) return to jacket temperature. temperatures), whereas the single pass allows only a 3 F (7 C) range. The larger range of the multipass means that less water has to be pumped to and through the absorber to supply a given heat input. This helps to compensate for the higher pressure drop of a multipass arrangement. If the maximum return water temperature to the engine jacket was 90 F (88 C), as quoted by some engine manufacturers, the situation becomes appreciably worse, as shown in Figure 8. With a 90 F (88 C) maximum engine jacket entering temperature (and therefore a 90 F [88 C] absorber leaving water temperature) the Inlet Temp. ( F) F 98 F maximum entering water temperature for a multipass absorber is 22 F (00 C), with a capacity factor of ~65%. Table 2 illustrates how sensitive the absorber capacity factor is to engine jacket entering water temperature, and also the value of using multipass machines. Both absorber derating, requiring installation of a larger absorber, and the larger hot water flows, requiring larger piping and pumps, Capacity Factor 40% Figure 7: Operating points to achieve 90 F (88 C) return to jacket temperature. 80 Single-Pass 90 F Outlet Temp. ( F) 00% 00% can make a significant difference in first cost. There are situations where single-pass arrangements will make sense, specifically where temperature ranges must be kept low or the water flow rate must be high for some other system-related reason. However, with engine coolant, the limited return temperature to the engine and the ability for an engine to generate high (250 F or more [2 C or more]) leaving water temperatures suggest that engine heat recovery is a problem best solved with a multipass chiller. Number of Passes Maximum Engine Jacket Inlet, F Maximum Inlet Temp., F Resulting Capacity Derating Factor Chiller Size Required to Deliver 00 tons, Tons Engine Flow Required For 00 Tons, gpm % 73% 86% % 87% 0% Table 2: Resulting derating situation and water flows for 00 tons (350 kw) of heat recovery cooling September 4 Building for the Future A Supplement to ASHRAE Journal S33

5 How Heat Exchangers Make the Situation Worse Designers often place a heat exchanger between the absorber and the engine jacket. Engine manufacturers may actually recommend this as it relieves them of any concerns about the capability of the engine water pump to handle the pressure drop through the absorber. Unfortunately, this is not desirable from an overall system standpoint. If the heat exchanger has a 0 F (5.5 C) drop, as is typical of shell and tube arrangements, and the maximum return temperature of the engine is 90 F (88 C), the maximum output temperature from the absorber becomes 80 F (82 C). As shown in Table 2, this results in a further derating of the absorber, increasing the design size of the absorber from 43 to tons (503 to 703 kw) just to effectively produce 00 tons (352 kw). Some designers also voice concerns about any leakage in the absorber generator heat exchanger potentially contaminating the engine coolant system. However, even when the absorber is running, the generator operates below atmospheric pressure, whereas the jacket coolant system is at or above atmospheric. When the absorber is shut down, the generator is far below atmospheric pressure. Any leakage in generator tubing would admit jacket water to the absorber, rather than leak bromide solution to the jacket water. Finally, although a heat exchanger dividing the two water flows allows the absorber s hot water flow pump and engine Pressure Drop, ft of water F 3 Pass 2 Pass pump to be in separate circuits, other ways exist of handling this, as described in the next section. Pumping Issues Moving jacket water through the absorption chiller directly involves overcoming the pressure drop within the hot water piping that runs through the generator. The longer the flow 90 F Pass 80 F Figure 8: Commonly used heat exchanger worsens situation Gallons Per Minute Figure 9: Pressure drop in a 60 ton (563 kw) (nominal rating) hot water absorption chiller. path, the greater the pressure loss. Therefore the pressure drop increases with the number of passes used. As shown in Table 3, moving from single to multipass machines both raises the pressure drop and lowers the water flow rate, resulting in similar power consumption. Pumping power does rise with lower water temperatures in either pass arrangement. However, it remains a small quantity compared to the cooling derived. The values in Table 3 do not include pumping needed to send the hot water from the engine to the absorber. A system with considerable distance between the engine and the absorber will consume more power. Some engine generators may be equipped with a pump on the engine. However, this pump will have been sized to move jacket water through a radiator and back to and through the engine. It may not be sufficient to handle pumping through the absorber. An additional pump may need to be added to circulate coolant through the absorber. Also, the pressure drops shown in Figure 9 are for water and will be higher than for the ethylene glycol water mixtures generally Number of Passes Maximum Engine Jacket Inlet, F Maximum Inlet Temp., F Resulting Capacity Factor 37% 73% 86% 87% 0% Table 3: Pump power across differing flow conditions. Chiller Size Required to Deliver 00 tons, tons Engine Flow Required For 00 tons, gpm Pressure Drop, ft Pump Power at Efficiency, hp S34 Building for the Future A Supplement to ASHRAE Journal September 4

6 ABSORPTION HEAT RECOVERY CHILLERS used in engine jackets to prevent radiator freeze-up in the winter. Control Valve Sizing and Control In most commercial building applications, cooling, heating, power (CHP) or cogeneration systems make the most economic sense when sized to cover about of a facility s electric load.,2 The generator operates nearly continuously to cover 40% to of the full electric load. The infrequent peak electric loads are covered by utility power. Properly connected, engine generators in the 32% efficiency range can provide enough heat to power, at most, 250 to 300 tons (880 to 055 kw) of singleeffect absorption chiller for each MW of generator installed. Commercial buildings require between 5 and 5 W/ft 2 (54 and 6 W/m 2 ). This includes all electric loads including electric cooling systems. Conversely, each 400 ft 2 (37 m 2 ) of building requires roughly ton (3.5 kw) of cooling. CHP systems in commercial buildings, as suggested in Table 4, feature the following: CHP systems tend to have more favorable economics in larger installations. In general, for a given climate and utility rate structure, the amount saved by the systems is relatively constant per square foot of building floor space. However, the first cost of the system declines precipitously with building size, making paybacks shorter for larger systems.,2 Engine generators are the most practical way of generating electricity in systems in the 0.5 MW to 2 to 3 MW range. This would cover buildings up to ~500,000 ft 2 (~ m 2 ), which includes the vast majority of commercial buildings. Systems serving larger floor space loads, where turbines may be more practical, would include only very large hospitals and collections of buildings on central heating and cooling systems. The emerging technology of microturbines may change this situation soon. The cooling provided by an absorption chiller operating on generator waste heat tends to cover typically about one-third of the peak cooling load. The remainder of the cooling needs must be provided by either conventional chillers and/or by supplementary firing of the absorber. F Max Dump Radiator Expansion and Pressurization Tank Figure 0: Layout showing control system and heat rejection. Given that the absorption chiller is operated on reclaimed heat from a generator, the absorber should be the first chiller operated (the lead chiller) whenever a cooling load is present. Other chiller capacity should then be brought on only as needed. Controlling the System Diagrams presented thus far have shown very simple systems. The next issue to handle is controlling the output of the absorber to match the needs of the cooling system. The standard method is to control the volume of hot water flow through the generator with a two-way or three-way control valve. Given that the overall water flow through the engine jacket must remain constant, and that some water flow should be maintained through the water-cooled silencer, the three-way arrangement lends itself to this system. In addition, during periods when the absorber is producing less than full cooling or is shut down, the unused heat from the engine jacket must be rejected to the environment. At this point, the dump heat radiator is introduced into the system, as shown in Figure 0. A cooling tower could also be used. Given that this cooling tower is also needed for the absorption chiller, this can be a practical arrangement for CHP systems. However, in colder climates, engine cooling to a cooling tower will require cooling tower operation in freezing weather, generally requiring a dry sump. Notice that an expansion and pressurization system also has been added. The water system must be kept at a pressure above T Chilled to Building Size, ft 2 00, ,000,000,000 Peak Electric 700 kw 3,500 kw 7,000 kw Cogen Size at 350 kw,750 kw 3,500 kw Size, tons Peak, tons Cost of Installed System $,400/kW $,/kw $800/kW Cost of System Per ft 2 of Building $5.50 $3.33 $2.80 Table 4: Approximate sizing of CHP systems in commercial buildings. September 4 Building for the Future A Supplement to ASHRAE Journal S35

7 the boiling point of the hottest water in the system. A water-cooled silencer (for exhaust gas heat recovery) equipment manufacturers diagram is shown in Figure 0. An exhaust gas diverter valve controls the water-cooled silencer. When the engine operates and some or all of the waste heat is not needed, the valve opens and sends exhaust gas around the heat exchanger. Even when this is done, some flow of water should continue to pass through the silencer to prevent boiling due to any possible diverter valve leakage. In Figure 0, the silencer flow bypasses the radiator, but does mix with the cooling flow. Therefore, the return to the silencer should not significantly exceed the temperature out of the engine jacket, even when heat usage is zero. Space and Heating Having handled the cooling issues, the more straightforward recovery for space and water heating should be incorporated. As previously mentioned, water jackets typically can produce hot water in the 240 F to 250 F (6 C to 2 C) range. The critical temperature is the allowable entering temperature to the water jacket, and engine manufacturers may limit this to below F (97 C). If there is no productive heat load that will lower the return water to F (97 C), much of the engine jacket heat will be wasted. This has a negative impact on cogeneration economics. Figure details an entire interconnection arrangement between an engine generator, an absorption chiller, and a heating delivery system. The absorber uses engine coolant directly with no intermediate heat exchanger to minimize derating. Using a heat exchanger for a building space heating system almost always is required. Building heating systems involve extensive piping, and, running engine coolant throughout the building generally is not practical. In addition, the highest temperature used for most heating application is 80 F (82 C), well below the maximum engine return temperature. When the space heating load is sufficient, all of the recovered heat can be used and the waste heat radiator remains inoperative. In addition, in larger commercial buildings, domestic hot water often is generated as part of the overall heating system. This single heat exchanger would then pick up the domestic hot water load, as well. < F T Heat Exchanger Single Effect Absorption Chiller Pump Radiator Operation Triggered by Leaving Temp. Exceeding F CWR CWS HWS 80 F Max. HWR 60 F Max. From To To Heating From Heating Figure : Possible connection arrangement between engine generator and heat recovery system. Low-Temperature Absorption Chillers Single-effect absorption chillers also can be built that operate, with little derating, on lower temperature hot water. These chillers use a different structure to the generator, allowing lower temperature hot water to be used more effectively. To date, there has been no American production of such equipment. Given a strong enough market in cogeneration systems, these chillers may become available in the future. Some foreign equipment of this type has been brought into the market, but this is beyond the scope of this article. 3 Conclusion Applying absorption chillers in conjunction with enginedriven cogeneration systems requires some thought about overall operation. Otherwise, the cost of the absorbers may become too high or the system may produce less cooling than expected. In cogeneration economics, recovered heat offsets some of the generator s fuel consumption by reducing boiler fuel consumption. With the rising cost of fossil fuels, maximizing the useful recovery of heat from a cogeneration system is more important than ever. References. Ryan, W., C. Haefke and M. Czachorski. 2. Evaluation of Commercial Markets for BCHP Applications, Gas Technology Institute. 2. Ryan, W. 2. Economics of Cogeneration. ASHRAE Journal 45(0): Century Single Stage Absorption Chiller, Product Specification Sheet. William Ryan, Ph.D., P.E., is a research engineer at the University of Illinois Chicago. S36 Building for the Future A Supplement to ASHRAE Journal September 4