1、OR-05-9-1 A CHP System Optimization with Microturbine Recuperation Control J.A. Parsons, PhD Member ASHRAE Bin Li ABSTRACT This paper documents an analytical study of a microtur- bine-based CHP system for a small commercial building. The flow exiting the microturbine turbine (exhaust) either enters
2、the hot side of the microturbine recuperator or is allowed to bypass this recuperator for variable recuperation. While bypassing reduces the microturbine thermal ejciency, there is better matching of the CHP thermal and electrical outputs to the building thermal and electrical loads, respectively. A
3、bsorption chilling, hot waterspace heating, anddomestic hot water heating are employed. The overall CHP system yearly performance is investigatedfor various CHP and microturbine operating modes. INTRODUCTION Building cooling, heating, and power systems, CHP (also called combined heat and power and I
4、ES - integrated energy systems), are applied to buildings that implement on-site or near-site power generation and utilization of the recovered exhaust heat for driving thermally activated equipment (TAE). As a prime mover, a microturbine can be u,sed to generate the electrical power and thermal ene
5、rgy. The thermal efficiency of a microturbine with recuperation can be up to about 30% while the efficiency without recuperation may only be 15%. The rest of the fuel thermal energy is used in TAE for building cooling, heating, domestic hot water, and/or humidification control. For a CHP system, the
6、 ideal situation is that the electricity is generated with high thermal efficiency and matches the building electrical load, and that the exhaust thermal energy generated by the prime mover, for instance a microturbine, matches the thermal energy and temperature needs of efficient TAE that best sati
7、sfy the building thermal load(s). This ensures maximum fuel efficiency, in the range of 80%. However, the performance of a CHP system for a given appli- cation is influenced by many factors including weather (inlet conditions to engine) and the design, capacity, construction, and control ofthe equip
8、ment. The performance map for a CHP system, the relationship between thermal energy and electrical output produced at maximuddesign and part-powedoff- design conditions, is of prime importance. However, more important is the instantaneous relationship between the build- ings electrical and thermal l
9、oads (if thermal energy storage is not considered or is not practical). For a given building the levels of thermal and electrical load change due to the time of the day, day of the week, and season of the year, occupancy schedules, and HVAC equipment schedules. Therefore, how close the produced elec
10、trical output and thermal energy match the buildings load map is another indication of efficient CHP system design. During operation the components of the CHP system operate at their individual design and off-design condi- tions. Therefore, adjusting or changing the CHP system component(s) to match
11、more occurrences of the building elec- trical and thermal loads is a desirable characteristic. For recip- rocating internal combustion engines, changing the compression ratio directly affects this engines thermal effi- ciency. For recuperated gas turbine engines, the recuperation rate (amount of exh
12、aust thermal energy transferred to the air exiting the compressor), the turbine inlet temperature, and the engine pressure ratio affect the thermal efficiency and maxi- mum network. For stirling engines, the engine maximum and minimum temperatures also drive the thermal efficiency. For variable recu
13、perated microturbine research, the authors are only aware of work by Jaffe (2003) sponsored by the Califor- nia Energy Commission. J.A. Parsons is an assistant professor and Bin Li is a graduate student in the Department of Mechanical Engineering, Mississippi State Univer- sity. 02005 ASHRAE. 779 Fi
14、gure 1 Microturbine with and without recuperation. Figure 1 shows a schematic and flow controls for a micro- turbine operating with and without recuperation. While this is not new microturbine technology, one goal of this study is to determine whether microturbine recuperation control can be used to
15、 optimize CHP system design and operation and, thus, reduce fuel consumption. The study is done in the following manner. First a building is chosen and the buildings electrical and thermal loads are determined, regardless of the buildings HVAC or CHP equip- ment. Next the electrical power consumed b
16、y the buildings HVAC or CHP equipment must be estimated and added to the building electrical load. Then the distribution of the instanta- neous electrical and thermal loads must be determined for the building-system operation, which is for a typical year. Then with the known loads, the CHP system eq
17、uipment can be selected to best match the building loads. There may be iter- ations back to the first step to properly account for system configuration and component size (capacity) changes. The next step is to model the CHP system, the building (thermally), the controls, and then perform a transien
18、t analysis for a year. While many studies use one-hour time steps, this study uses smaller, l/lOth hour steps for the following reasons. The CHP and typical HVAC equipment is selected to satisfy, or almost satisfy (up to 99% of the time), the maximum expected thermal loads. When the building loads a
19、re smaller than the maximums (which is most of the system operating time, also most of the year), the equipment will run at part maximum conditions and/or cycle on and off in order not to overcool or overheat the building. A typical control for turning on and off equipment is a thermostat sensing bu
20、ilding air temperature. Usually a dead band is incorporated into the ther- mostat to control cycling. Also, by using small time steps, transient thermal response of the building itself is included in the analysis. The sizing of equipment is fine tuned (capacities adjusted) to just satis the thermal
21、loads by observing how the building temperature approaches the desired setpoint temper- ature on maximum load days (likely the coldest and warmest). The simulation is run in three modes: (1) at maximum microturbine electrical power output, (2) at electrical load following, and (3) at thermal load fo
22、llowing. For the first -1 Figure 2 CHP system components. 3 mode, the controller output is constant and set at the nominal maximum of 30 kW. For the second mode, the sum of the instantaneous electrical loads for the building (lights, recep- tacles, and air circulating fan), the chiller, the cooling
23、tower, and all the loop circulating pumps is the controller output. For the third mode, the thermal energy inputs to the chiller, space heating, and hot water are satisfied. However, the boiler may also be on for high thermal energy load conditions. Also, the effect of microturbine recuperation is c
24、alculated in three modes, for a recuperated microturbine, for an un-recuperated microturbine, and for variable recuperation. For variable recu- peration, the total, instantaneous fuel flow rates for both a recuperated and an un-recuperated microturbine in combina- tion with the boiler are calculated
25、 and the smaller, total fuel flow rate combination is used in the simulation for the current time step. The primary objective of this study is to determine if vari- able recuperation can conserve fuel for a CHP system. Secondly a method is presented to select prime movers for and to optimize the CHP
26、 system performance. Also, the effect of simulation time steps smaller than one hour is studied. BUILDING AND CHP SYSTEM DESCRIPTIONS Overall System For this study a small commercial building is selected, roughly sized for a commercially available 30 kW microtur- bine that produces the electrical ou
27、tput and thermal energy. Figure 2 is a schematic of the CHP system components. The microturbine exhaust gas temperature is higher than ambient, creating a potential difference for thermal energy flow to the TAE. This exhaust is directed to an adjustable flow splitter (diverter) and a heat exchanger.
28、 The exhaust flow then recom- bines after the diverter and heat exchanger and is discharged to the atmosphere. The exhaust thermal energy is transferred in this heat exchanger to the main water loop. After the water in the main water loop leaves the heat exchanger, the water flows to a temperature-c
29、ontrolled boiler. From the boiler the water is routed in parallel to the absorption chiller, the space heating heat exchanger, and the hot water heat exchanger. The 780 ASHRAE Transactions: Symposia absorption chiller has an external, outside cooling tower, and a chilled-water loop to the building.
30、The cold side of the space heating heat exchanger forms the supply for the space heating water loop to the building. The hot water heat exchanger trans- fers thermal energy to the domestic water supply for the build- ing. The present work is an extension of Parsons and Li (2004), the same building a
31、nd similar CHP system, which investigated CHP fuel consumption and operating economics. The buildings thermal requirements are satisfied by the chilled water and space heating water loops. These loops provide energy to or remove energy from the thermal model of the building. This building model incl
32、udes the materials and construction of the walls, floor, ceiling, attic space, window, orientations, etc., and determines the building thermal tran- sient response, which is how the temperatures of parts of the building model vary and also how the inside air temperature varies, which is then used as
33、 an input to the thermostat. The thermostat and controller send outputs that are control signals to the microturbine, the diverter, the space heating, and the absorption chiller. TRNSYS (version 15.1) is used to perform the time-dependent, transient analysis performance of the CHP components, contro
34、ls, and building (Klein et al. 2000). For the building, electrical power is first supplied by the microturbine and then, if necessary, by the grid. The electrical loads in this model are: the absorption chiller, the cooling tower fan, the absorption chiller-cooling tower loop pump, the building inte
35、rnal loads (lighting, receptacles, and circulating fans), and the circulating pumps for the main, chilled, hot water, and space heating water loops. The controller turns on the proper components based on the thermostat input, the ther- mostat settings (set back, set up), and the building schedules.
36、Also, the grid accepts any excess electrical power generated by the microturbine and not consumed by the loads. All of the heat exchangers indicated in Figure 2 perform with 90% effectiveness for the transfer of thermal energy from the hot side to the cold side. Target water loop temperatures are di
37、scussed below. Based on CHP heat exchanger practice, an 1 1.1 “C (20F) temperature difference between the heat exchanger hot side outlet and the cold side outlet is used in this model. The absorption chiller component operates at the chillers nominal hot side water flow rate and, thus, determines th
38、e chillers outlet temperature to the main water loop. However, the flow rates to the space heating heat exchangers are adjusted to provide fixed amounts ofheating. The flow rate to produce hot water satisfies the instantaneous hot water demand. Building Commercial building #3 from the US Core Buildi
39、ng Stock Library is used in this study. This building has a single floor with 743 square meters (8000 square feet) (Briggs et al. 1987, 1992). The TRNSYS thermal building model for build- ing #3 (TESS 2003) is complete with weather data, floorplans, all thermal details, occupancy schedules, HVAC equ
40、ipment schedules, thermostat setpoints, and electrical power loads. The original location for building #3 is El Paso, Texas, but for this study two different locations are used, Birmingham, Alabama, and Minneapolis, Minnesota. Birmingham repre- sents a hot and more humid climate while Minneapolis ha
41、s colder winters, and, thus, significantly higher space heating loads. The simulations are performed for one year using typi- cal mean year weather data, TMY2. The building is run in the temperature control mode and not in the energy control mode. With the temperature control mode of operation, the
42、thermal energy absorbed by the chiller and the energy supplied by the space heating water loops are inputs to the building model. The building has nine floor zones. For each floor zone, the building model then determines the transient temperature response due to these thermal ener- gies and the buil
43、ding loads. The building reacts with thermal resistance and thermal capacitance. The thermal energies into or out of the building model are floor-area weighted and the cooling energy is split 30% for latent (to remove moisture) and 70% for sensible (to reduce air temperature), while the heating ener
44、gy is all sensible. A floor-area weighted average temper- ature is the input to the single thermostat for the building. An air-circulating fan, sized at 4 horsepower (hp), is on according to the building control schedules and thermostat output. A domestic hot water schedule is an additional thermal
45、load to the CHP system and is added to the building model. However, the domestic hot water represents a small fraction of the over- all energy needs of the CHP load in the simulation. Thermal energy from the domestic hot water system does not contribute to the building thermal response. Microturbine
46、 A commercially available 30 kW microturbine is modeled using curve fits of published manufacturers data. However, since the lowest part-power data only went down to 2 kW, straight-line interpolations/extensions are spliced in to cover the range from O to 2 kW. Since the microturbine will operate fo
47、r only a short time in this power range, this is the most practical way to fully complete the definition of the microturbine. The inputs to the microturbine model are power demand, altitude (for inlet pressure), ambient temperature, pressure drop for the exhaust side of the microturbine, fuel type,
48、and fuel pressure (natural gas at 55 psia). The outputs are electric power produced, efficiency, exhaust flow rate and temperature, and fuel flow rate. Diverter and Heat Exchanger The diverter operates to channel sufficient exhaust flow through the hot side of the heat exchanger for the cold side ou
49、tlet temperature (on the main water loop side) to be 88.1“C (i 90F). If there is no thermal load (chiller, space heating, and hot water all off), then the diverter channels the exhaust directly to the atmosphere, bypassing the heat exchanger. For many conditions, the cold-side outlet temperature is below 88.1“C. The heat exchanger hot-side outlet temperature only goes down to 99.2“C (2 10F) to keep an 1 1.1 “C (20F) differ- ASHRAE Transactions: Symposia 781 entia1 between the two outlet temperatures. This hot side outlet temperature is also above the 57.6“C (135F) minimum temp
