Assessing Value Of CHP Systems

Transcription

1 The following article was published in ASHRAE Journal, June 24. opyright 24 American Society of Heating, Refrigerating and Air- onditioning 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. Assessing Value Of HP Systems By Steve Fischer R ecent interest in combined cooling, heating, and power (HP) has been driven by desires to exert a little control over what facilities pay for electricity and to ensure uninterrupted power when the electricity grid is overloaded. Governments and private companies are recovering waste heat from generation and using it to offset what would have been fossil fuel purchases for space heating, production of domestic hot water and electricity for air conditioning chilled water. Unfortunately, no significant information has been developed about the value of recovered waste heat to the individual building owner or operator. 1,2,3 Somasundaram et al. published a series of nomographs 4 as one technique for assessing the economic potential of HP systems. This article presents a second approach that uses estimates for operational savings and simple payback using data for the amount of recoverable heat from electric generators, generator fuel consumption, average gas and electric rates, and performance parameters for boilers and chillers. Figure 1 is a graph of manufacturer s data for generator heat rate and the amount of recoverable waste heat. 5,6,7 The figure shows recoverable heat per kwh of electricity produced. It includes data for internal combustion engine-driven About the Author Steve Fischer is a senior research engineer at Oak Ridge National Laboratory in Oak Ridge, Tenn. 12 ASHRAE Journal ashrae.org June 24

2 generators (including engine jacket, lube-oil, and exhaust heat), gas-turbine and microturbine-driven generators, and fuel cells. The data are roughly linear in heat rate. As the heat rate goes down (and efficiency goes up), the amount of recoverable heat goes down. These data also appear in Table 1 with cost data and a separation of waste heat between total recoverable energy and high quality recoverable heat from exhaust gas that can power a double-effect absorption chiller. 8,9 Recovered heat from a HP system generally is less than what is theoretically recoverable because of non-coincidence of power generation and thermal loads or an absence of applications for low grade heat.* What the recovered heat is worth to the building operator, and whether or not it is sufficient to offset the investment and cost of operation, depends on how the heat would be used. This article provides a few straightforward equations that can be used in a coarse screening to determine if there is sufficient merit in using HP at a site to warrant a more rigorous analysis by site engineers or an architecture/engineering firm. Space Heating and Hot Water Depending on local energy costs, heat recovered to be used in place of purchased fuel to provide space heating or domestic hot water can offset a large part of the generator fuel cost. The avoided fuel purchases can be estimated by dividing the heat recovered from engine exhaust, jacket cooling water, or the lube oil cooler by the efficiency of the heating or hot water system, as shown in Equation 1. heat & hot water Q& recovered = 1,, η fuel boiler Q & recovered =recovered heat used to offset boiler or water heater fuel (Btu/kWh generated ), fuel =average cost of boiler fuel ($/Million Btu), η boiler =average boiler efficiency heat & hot water =avoided cost of boiler fuel ($/kwh generated ). The terms in Equation 1 are defined in Table 2. Figure 2 shows four curves for the value of waste heat used for heating or hot water at different heating system efficiencies across a range of fuel prices. Naturally, as oil or natural gas prices go up, the value of the recovered waste heat goes up. Also, as the efficiency of the heating system goes up, less fuel is needed to meet a fixed heating load, so the value of waste heat used to meet that load would go down. (1) hilled Water Production The same approach is used to estimate the value of recovered waste heat used to produce chilled water with an absorption chiller. In this case, the avoided energy cost is what it would cost to operate an electric chiller to produce the same amount of chilled water, as shown in Equation 2. chilled water gop Q& = recovered 12, electricity June 24 ASHRAE Journal 13 η chiller η chiller (2) gop=absorption chiller efficiency (nondimensional), =efficiency of the electric chiller (kw/ton) that would have been used instead of a waste-heat driven absorption chiller, electricity =average cost of power ($/kwh), chilled water =avoided cost of chiller power ($/kwh generated ). Figure 3 shows curves for the value of chilled water using steam or hot water-driven double-effect absorption chillers (gop = 1.1) for several different electric chiller efficiencies across a range of average electric costs. Again, as expected, the value of the waste heat is higher in areas where electricity costs are high, and lower in regions with relatively cheap electricity. If heat driven absorption chillers are used in place of older, less efficient electric chillers, the waste heat is of greater value than if the alternative chilled water system uses higher efficiency electric chillers. Finally, double-effect chillers produce approximately 6% more chilled water for each unit of thermal input than do single-effect chillers, so waste heat used in double-effect chillers has a higher value than it does when used in a singleeffect chiller (not shown). The benefits of higher efficiencies for double-effect chillers must be considered with their requirements for high-grade heat. If only the high temperature heat is useful for producing chilled water, then the recovery rates shown in Figure1 may be too high, particularly for internal combustion engine-based HP systems and calculations should be performed using only the high grade waste heat for Q & recovered. The capability of singleeffect chillers to use lower quality heat can lead to instances where they can provide greater chilled water capacity at lower cost than recovered heat-fired double-effect chillers. While reductions of electric utility demand charges resulting from onsite generation are included in the calculations through the average cost of power, the analysis does not account for electric demand reductions (and cost savings) that would occur as a result of using a smaller electric chiller plant when the HP system includes absorption chillers. onsequently the results displayed in Figure 3 are somewhat conservative. Detailed analysis of one site with demand charges of $8.77/kW and consumption costs of $.37/kWh during on-peak periods * In this article, the term waste heat is used when referring to heat rejected by a turbine or engine in the exhaust gases, jacket cooling water, and lube oil cooler; recoverable heat is used to refer to heat that can be recovered from the waste heat streams whether or not an end use is available for this energy; recovered heat is the portion of recoverable heat that can be used effectively to satisfy site thermal loads. Examples and figures assume that all of the recoverable heat can be used effectively (β=1 in Equation 3) for a coarse screening of economic potential. Parameters β 1 and β 2 are used to apportion recovered heat between space heating and chilled water; parametric results for when recovered heat is less than the recoverable amount are presented in Figure 4.

3 Heat Recovery (Btu/kWh) 1, 8, 6, 4, 2, Recip. Engine Generators Gas Turbines Microturbines Fuel ells Value of Heat ($/kwh Generated) $.2 $.15 $.1 $.5 Boiler Efficiency (9 hours per day) and $.32/kWh during off peak (15 hours per day) showed actual savings to be about 8% higher than those computed using Equation 3 with the average cost of electricity. HP Savings Factor Equations 1 and 2 can be put together to develop an estimate of the savings (or losses) from a HP system for each unit of electricity produced: HP = γ electricity 1 αheat rate fuel + β 1 heat & hot water 2 1,, standby O&M + ( γ) export H [ β + β ] chilled water where γ = fraction of power generation consumed on site, O&M = generator operating and maintenance cost ($/kwh generated), standby = electric utility standby charges ($/kw per year), = price received for exported power ($/kwh), α export heat rate 5, 1, 15, 2, Heat Rate (Btu/kWh, HHV) = generator heat rate (Btu/kWh generated), β = fraction of annual recoverable heat that is recovered and used for heating or chilled water, β 1 = fraction of annual recovered heat used for heat and hot water, and β 2 = fraction of annual recovered heat used to produce chilled water. Equation 3 combines the avoided cost of purchased electricity, generator O&M costs, prorated electric utility standby charges, the value of exported power, and the cost of generator fuel with the values of recovered heat used for space heating, potable hot water, or chilled water. Sample values for the terms in Equation 3 are in Table 2. (3) Generator Type Recip. Engine 1 kw 8 kw 3, kw Gas Turbine 1, kw 5, kw 1, kw Microturbines 3 kw 6 kw 2 kw Fuel ell 2 kw Example The calculations in Equations 4 through 6 use sample data for a 1, kw gas turbine based HP system producing steam for heating and chilled water from a 5 ton absorption chiller. The value of energy recovered for space heating or to produce hot water is approximately: Similarly, recovered heat used to produce chilled water is worth about: chilled water Installed ost $/kw ~$3,5 $.15 + $.42* 9,422 4,5 *Stack replacement estimated on 95% availability, 5% of initial cost and five-year stack lifetime. Table 1: Engine-generator cost and energy parameters. 14 ASHRAE Journal ashrae.org June 24 $. $1,39 $975 $85 $1,64 $1,77 $964 $1,87 $1,87 $1,87 $. $5. $1. $15. ost of Fuel ($/Million Btu) Figure 1 (left): Heat recovery for on-site power generation. Figure 2 (right): Value of recovered waste heat used for space heating or potable hot water. 7,82 Btu kwh generated heat & hot water = 1,, Btu Million Btu.72 $5/Million Btu = $.54/kWh generated (4) 1.1 7,82 Btu kwh generated.85 kw ton = 12, Btu ton h [ $.85 kwh] = $.52/ kwh generated (5) O&M osts $/kwh $.164 $.17 $.13 $.96 $.59 $.55 $.113 $.113 $.113 Heat Rate (HHV) Btu/kWh 12,126 11,5 1,158 15,6 12,375 11,75 13,36 13,36 13,36 Heat Recovery (Btu/kWh) Total High Quality 5,683 4,325 2,99 7,82 5,62 5,283 4,5 4,5 4,5 3,5 1,85 1,45 7,82 5,62 5,283

4 Variable α heat rate β β 1 β 2 HP γ η boiler η chiller chilled water demand e export Definition Generator Heat Rate (Btu High Heating Value/ kwh Generated), e.g., 15,58 Fraction of Recoverable Heat That an Be Used to Offset Purchases of Fuel or Electricity, e.g., 1. Fraction of Recovered Heat Used for Heat or Hot Water on an Annual Basis, e.g.,.5 Fraction of Recovered Heat Used to Produce hilled Water on an Annual Basis, e.g.,.5 Avg. Savings From HP Operation ($/kwh Generated), e.g., alculated Value of $.47 Fraction of Generated Electricity That Is Used On- Site, e.g., 1. Avg. Annual Efficiency of Boiler for omputing Avoided Boiler Fuel Purchases, e.g.,.72 Avg. Annual Efficiency of Electric hiller for omputing Avoided Power Purchases (kw/ton), e.g.,.85 Value of Recovered Heat Used to Produced hilled Water ($/kwh generated), e.g., $.52 alculated Value) Annual Electricity Demand harges ($/kw), e.g., $16 Annual Avg. Electric Use Rate ($/kwh, no Demand harges), e.g., $.42 Price Received for Exported Power ($/kwh), e.g., $.25 Table 2: Nomenclature and sample data values. Variable electricity fuel heat & hot water O & M standby gop H N P HP System Q & recovered Definition Annual Avg. ost of Electricity Including onsumption and Demand harges ($/kwh), e.g., $.85 Annual Avg. ost of Fuel ($/Million Btu), e.g., $7. Value of Recovered Heat Used for Space Heating Or Hot Water ($/kwh generated), e.g., $.54 alculated Value Annual Avg. Generator O&M osts ($/kwh Generated), e.g., $.96 Electric Utility apacity Standby harge ($/kw/y), e.g., $3 Absorption hiller Efficiency (Dimensionless), e.g., 1.1 Annual Generator Operating Time (h/y), e.g., 8,4 Simple Payback (years), e.g., alculated Value Of 4.6 HP System Net Installed ost Including redits For a Non-HP Alternative, Rebates, and Equipment Subsidies ($/kw Electrical apacity), e.g., $1,87 Generator Heat Recovery Rate (Btu/kWh Generated), e.g. 7,82 Assuming that all of the recoverable energy can be used to offset purchases of electricity and fuel, and that 5% of the recovered heat is used for heating or hot water and 5% is used to produce chilled water, then: HP = 1. $.85 / kwh $.96 / kwh $3/kW/year $.25/ 8,4 h/y 15,6 Btu/kWh 1,, Btu/Million Btu $ 5/ Million Btu ( ) kwh $.54/kWh $.52/kWh = $.47/kWh generated (6) If the two terms in Equation 6 for the values of recovered heat are omitted, representing distributed generation (DG) instead of HP, the savings factor would be negative. A HP system is profitable under these conditions while a DG system would lose $.6 for each kwh generated. Simple Payback Equation 7 represents the simple payback for a HP system using estimates for the installed cost from Table 1 and the HP savings rate from Equation 3. PHP System N = H HP (7) P HP System HP N = simple payback period (years), = net cost of HP system including installed cost of all equipment less cost of baseline HVA system, avoided costs for emergency or backup power, and rebates or equipment subsidies, H = generator operation (h/year), = net HP savings ($/kwh generated). Simple payback for the example compared to a do nothing baseline above is about 4.6 years ([$1,64 + $23]/ [84 $.42]) including a 5-ton (1759 kw) doubleeffect absorption chiller at $45/ton ($115.15/kW). 9 The payback would be lower using if rebates or equipment subsidies are available or if the comparison is against an upgrade to an electric chiller plant without HP. Equations 3 and 7 can be used to generate graphs of simple payback to illustrate the sensitivity of payback to variations in energy costs. Figure 4 shows the sensitivity of simple payback for the example above for average electricity costs of $. to $.25/kWh and fuel costs of $. to $25./Million Btu. Each red curve shows the calculated payback periods for the gas-turbine-based HP system in the example with 1% of the recoverable heat used for heating and chilled water at a fixed average cost of electricity. Payback at the energy costs in the example is 4.6 years. Payback increases to 7.4 years at a gas rate of $7/Million Btu and electric rate of $.85/kWh. Payback at $.7/kWh and $5/Million Btu for gas is 7.3 years, assuming that 1% of the June 24 ASHRAE Journal 15

5 Value of Heat ($/kwh Generated) $.2 $.15 $.1 $.5 Electric hiller Efficiency 1.1 kw/ton.8 kw/ton.65 kw/ton.57 kw/ton $. $. $.5 $.1 $.15 $.2 Avg. ost of Electricity ($/kwh) Simple Payback in Years $.5 $.1 $.15 5% 75% 1% onstant ost Power urrent ost Fuel urrent ost Power urrent System $.2 $.25 $ $5 $1 $15 $2 $25 Avg. ost of Fuel ($/Million Btu) Figure 3 (left): Value of recovered heat to power a double-effect absorption chiller. Figure 4 (right): Simple payback at fixed electric costs for 1, kwh gas turbine HP system using 5% of recovered heat for space heating and 5% for chilled water. recoverable heat can be used. The yellow curve shows payback for the assumed cost of power, $.85/kWh across a range of fuel costs. The vertical line with triangular markers shows the payback periods when all of the recoverable heat can be used (1%, 4.6 years) and when it is not possible to use all of the recoverable heat (5%,1.5 years and 75%, 6.4 years). Higher capacity HP plants can benefit from economies of scale for equipment costs and higher efficiencies with larger generators, but they need to be matched with site power requirements and thermal loads to ensure high operating hours and high usage of recoverable heat. Peak shaving installations can benefit from higher average electricity costs, but lower annual operating hours can still lead to long payback periods. The shapes of the curves in Figure 4 are highly dependent on the allocation of recovered heat between heating and cooling end uses, the efficiencies of the equipment being replaced by recovered heat-powered equipment, and to a lesser extent on the size of the generator and chiller installed. P spreadsheets can generate similar graphs for alternative allocations of heat and equipment sizes. Demand harges The algorithms presented are based on using the average cost of electricity including both the energy or use charge and demand charges. Demand charges could be used explicitly by defining the cost of electricity as in Equation 8. electricit y = e + demand = average electricity energy charge ($/kwh) and e demand = annual electricity demand charges ($/kw). Equation 8 provides an explicit credit for demand charge savings from on-site generation. More savings could be achieved from reduced demand from substituting recovered heat powered absorption chillers for electric air conditioning or water chillers. H (8) Advertisement in the print edition formerly in this space. 16 ASHRAE Journal ashrae.org June 24

6 onclusions The equations and data presented here can be used to perform a screening of sites to determine whether a more rigorous evaluation of HP could be worthwhile. HP will not be attractive at some sites, even with the idealistic assumption that 1% of the recoverable heat can be used ( β =1) to offset purchased energy. Sites with marginal paybacks may be attractive if more of the recovered energy can be used for heating (increasing β 1, as in the example) or cooling (increasing β 2 ). Equations 1 and 2 can help guide these choices for the greatest economic benefit (i.e. heating or cooling) while information about thermal loads from boiler and chiller logs can aid in estimating the fraction of recoverable heat that can be used effectively in each application. Some applications of recovered energy use only high quality heat, in which case care must be taken either to reduce the fraction of recovered heat that can be used effectively through the parameter or by specifying a heat recovery rate, Q & recovered, that only reflects high quality heat from the engine exhaust. Estimates of the value of avoided purchases of electricity are somewhat conservative because they are based solely on average cost of power and do not include the value of reductions in demand when the HP plant substitutes recovered heat-fired absorption chillers for some or all of the electric chiller capacity that would be required without HP. Advertisement in the print edition formerly in this space. References 1. owie, M., et al. 22. HP for buildings: the challenge of delivering value to the commercial sector. Proceedings of IMEE22, ASME International Mechanical Engineering ongress and Exposition. 2. Fischer, S. and J. Glazer. 22. HP self analysis. Proceedings of IMEE22, ASME International Mechanical Engineering ongress & Exposition, IMEE Hadley, S., et al. 22. Analysis of HP Potential at Federal Sites, ORNL/TM-21/ A simplified self-help way to size small-scale cogeneration systems. ogeneration Journal 4(4): Kelly, J. 23. Why HP is still an option in today s gas price market. Presented at AGA s 23 National Accounts onference. 6. Fairbanks Morse, Inc., ogeneration, product literature, WG 3M- 587, File No Solar Turbines. 2. Solar Turbines: Heat Recovery Typical Performance, DSHR-TP28/1. 8. Onsite Sycom Energy. 2. The Market and Technical Potential for ombined Heat and Power in the ommercial/institutional Sector. 9. Dorgan,., S. Leight, and. Dorgan ASHRAE s New application guide for absorption cooling/refrigeration using recovered heat. ASHRAE Journal 37(7): June 24 ASHRAE Journal 17