A TEST FACILITY FOR DISTRIBUTED COGENERATION: EXPERIENCES ON A MICROTURBINE AND CHILLER BASED PLANT

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1 A ES FACILIY FOR DISRIBUED COGENERAION: EXPERIENCES ON A MICRO AND CHILLER BASED PLAN Stefano BANEA, Stefano BARSALI University of Pisa - Italy s.banetta@libero.it, barsali@dsea.unipi.it INRODUCION In recent years, research and industry have made a strong effort in making cogeneration a competitive alternative to the historically consolidated processes for energy supply. One of the key factors is the development of distributed generation plants having a reduced size, which enable the operators to exploit cogeneration affording reduced investments thanks to the small extension of the heat distribution grid. In several cases the cogeneration plant can directly replace the boilers installed for heat supply thus avoiding the development of a specific connection. o test some of the available technologies and check the possibility of a central and remote control, the University of Pisa has installed a cogeneration plant in a decentralized laboratory of the Faculty of Engineering. PLAN DESCRIPION he plant mainly consists of a 45 kw micro-turbine and a double stage absorption chiller/heater. he chiller is rated 176 kw of chilling power and 161 kw of heating power and uses water as refrigerant and a Lithium Bromide water solution as absorption fluid (see the photograph in Fig. 1) [1]. he test facility also includes some simulators of the thermal and electrical loads in some possible configurations (heating or cooling, grid connected or stand-alone operation), ES URBOALERNAOR ALERNAOR POWER CONDIIONING UNI CHILLER / HEAER OUPU PARALLEL SWICH LOCAL ES / HEAED INERFACE SWICH REMOE Fig. 2. Principle scheme of the microturbine and chiller based plant and a connection to the gas distribution grid. So a 2kW water/air heat exchanger, a 33kW cooling tower, a 5kW resistor bank and a gas compressor complete the installation. he principle scheme of the plant is drawn in Fig. 2. One of the most innovative characteristics of the plant is the use of a gas fired absorption chiller where the burner has been removed and replaced with the exhaust pipe of the microturbine. he exhaust temperature can reach temperature values close to 7 C thus giving the chiller good chances to have a high Coefficient Of Performance (COP). Such a high temperature is obtained at the expenses of the electric efficiency of the microturbine. In fact no heat recuperator is installed and no more than a 15% electrical efficiency is expected. he first phase of testing (partly completed) has the following purposes: assessing the energy balance of the overall plant and evaluating possible improvements; assessing the electrical performances of the microturbine; MAIN GRID Fig. 1. An overall view of the plant: the microturbine on the left, the chiller on the foreground and the cooling tower in the background. he normal stack and the by-pass stack are clearly visible. CIRED23_AVpaper_UPI_Barsali.doc Academic Village Paper No - 1 -

2 checking the inverter interfacing with the grid and its response to grid disturbances. ENERGY BALANCE Main purpose of the preliminary characterization of the plant was the evaluation of the electric and thermal energy fluxes. In Fig. 3 a schematic representation of the location of the dedicated diagnostic devices can be found. he microturbine adopted in this plant as a constant speed control. So, when the turbine power demand decreases from the rated value, the internal temperatures rapidly decrease too because of the constant amount of air drawn by the microturbine compressor and of the reduced amount of fuel. his effect makes the turbine working on a less efficient thermodynamic cycle. Variable speed turbines have a more constant efficiency profile, especially if a temperature control is implemented. On the other hand the turbine installed has a very fast response to sudden load steps (see forward). Chiller characterization Q,Q 5 EVAPORAOR GENERAOR,Q REMOE (FACULY) In the cooling mode, the absorption chiller operates as a heat pump powered by the heat made available by the turbine exhaust, in order to chill the water in the thermal load circuit. Heat pump cycle is closed dissipating heat via the cooling tower water circuit. During the heating mode operation, the chiller acts as a heat exchanger between the turbine exhausts and the water which supplies the thermal load. he cooling tower circuit is excluded.,p COMPRESSOR 1,P,Q ALERNAOR POWER CONDIIONING UNI URBOALERNAOR CHP PLAN LOCAL Fig. 3. Scheme of diagnostic equipment location (Legend: temperature, P pressure, Q flow rate, V voltage, W power) he dashed lines represents the borders of the CHP plant, having air and fuel as input and thermal power (heated/chilled water), exhaust gas and electric power as output. Microturbine characterization Microturbine characterization was aimed at evaluating fuel consumption, exhaust gas temperature (EG) and exhaust mass flow rate for different power output values. Output power levels from to 4 kw, with a 5 kw interval, were held constant for some minutes. he total energy generated in the time interval was correlated to the relevant fuel consumption obtaining the chart of Fig. 4. V,W V,W MAIN GRID Chilling operation. Chilling mode tests were carried out raising gradually the microturbine power output and consequently the EG. Fig. 5 shows some of the data recorded during the transient. hey refer to the five measuring points shown in Fig. 3. Up until t=2s in the graph, the cooling tower circuit was closed and so the real chilling effect starts after the pump was activated. At the end of the transient period, the values reported in ab. I were acquired and the energy fluxes shown in Fig. 7 were calculated. he turbine exhaust gas has a power content of kw, part of which is lost in the duct before the chiller and part remains unused in the stack residual. he remaining kw are exploited by the chiller to generate chilled water. he chiller energy balance shows that further 5.4kW are lost in the circuits of the chiller itself. he resulting chiller COP is about.58, while a value closer to 1 was expected. One of the possible reasons for this low COP is the 95 C fall in exhaust gas temperature due to the mentioned heat lost in the exhaust duct between turbine and chiller. As is clearly shown in the photograph of Fig. 1, no thermal insulation is presently installed in such section of the exhaust duct. his makes the resulting gas temperature lower than the design value, thus possibly causing the detriment in COP value. 14 Efficiency [%] Power [kw] Fig. 4. Microturbine efficiency vs. power chart Heating operation. In heating mode, the chiller works essentially as a classic heat exchanger. he test procedure was the same of the chilling mode and Fig. 6 shows part of the data recorded at the first four measuring points highlighted in Fig. 3. he turbine power demand was progressively increased, and the exhaust temperature grew together with the heated water one. he values reported in ab. II were acquired after the initial transient conditions, and the energy fluxes shown in Fig. 8 were calculated. In this case, kw out of 289.5kW are exploited by chiller/heater to generate heated water. In this case the chiller circuits loose 21.1kW out of the kw injected by the exhausts. CIRED23_AVpaper_UPI_Barsali.doc Academic Village Paper No - 2 -

3 Gas temperature [ C] urbine exhaust - 1 Chiller inlet - 2 Chiller outlet - 3 Cooling water - 5 Chilled water ime [s] 13 1 Fig. 5. Part of data recorded during a chilling mode test. Curve numbers refer to the measuring points shown in Fig Water temperature [ C] Gas temperature [ C] urbine exhaust - 1 Chiller inlet - 2 Chiller outlet - 3 Heated water ime [s] Water temperature [ C] Fig. 6. Part of data recorded during heating mode test. Curve numbers refer to the measuring points shown in Fig kw 5.4 kw CHILLER LOSSES 21.1 kw CHILLER LOSSES EVAPORAOR kw CHILLING POWER EVAPORAOR kw HEAING POWER INPU kw GENERAOR DUC LOSSES 45.7 kw kw POWER YIELD BY 55.4 kw RESIDUAL INPU kw GENERAOR DUC LOSSES 46.1 kw kw POWER YIELD BY 47.6 kw RESIDUAL POWER CONEN kw NE OUPU 38. kw POWER CONEN kw NE OUPU 36.9 kw ALERNAOR PCU ALERNAOR PCU GENERAL LOSSES 9.4 kw AL LOSSES 7. kw GENERAL LOSSES 9.4 kw AL LOSSES 7. kw Fig. 7. Energy fluxes in chilling mode Fig. 8. Energy fluxes in heating mode ABLE I-Overview of the main parameters acquired during a cooling mode test urbine Exhaust gas Chilled water Cooling water Fuel Exhaust gas Power flow rate emp inlet emp outlet emp inlet emp outlet emp inlet emp outlet emp (kw) (g/s) (kg/s) ( C) ( C) ( C) (m 3 /hr) ( C) ( C) (m 3 /hr) ( C) ( C) ABLE II-Overview of the main parameters acquired during a heating mode test urbine Exhaust gas Heated water Fuel Exhaust gas Power flow rate emp inlet emp outlet emp inlet emp outlet emp (kw) (g/s) (kg/s) ( C) ( C) ( C) (m 3 /hr) ( C) ( C) he corresponding overall efficiency of the plant (intended as the ratio between the useful power, electrical plus thermal, and the fuel power) is 61.8%, while the chiller efficiency as heat exchanger is as high as 89%. Also in this case about 46 kw are lost in the exhaust duct between the turbine and the chiller, so a thermal insulation might improve the overall efficiency. CIRED23_AVpaper_UPI_Barsali.doc Academic Village Paper No - 3 -

4 AL PERFORMANCES AND CHARACERISICS he microturbine turboalternator is a high frequency generator (close to 4kHz), so the electric power generated has not the right parameters to be injected in the grid. A power conditioning unit (a series of a rectifier, a chopper and an inverter) decouples the generator from the grid which, in turns, is fed at the proper frequency. Up until the turbine control succeeds in keeping the voltage on the DC section of the power conditioner within a given range, the AC side generator response only depends on the inverter control. he ability of the turbine to do this depends on the choices made in the design of the turbine control system. In fact the alternator is a permanent magnet machine and its voltage output depends on the electrical load and on the rotational speed. he lower the speed and the higher the load, the lower the voltage output. A constant speed control gives a good decoupling between the turbine behaviour and the AC side response, while temperature loops have a slower response even if ensure a more constant efficiency throughout the operational power range. Load steps he electrical performances of the microturbine can be evaluated in terms of the response to a load step when feeding a load separately from the mains. he turbine installed in this test facility has a speed control loop aimed at keeping the turbine speed at its rated value (116 krpm). he overall response to a load step is extremely fast. Fig. 9 shows voltage and current at inverter terminals for the insertion of two resistive loads rated 22 e 38.5kW. he voltage drop is recovered within a few cycles and, thanks to the decoupling between the alternator and the AC grid, the turboalternator response doesn t affect the AC side behaviour. he temporary speed reduction was not monitored in these tests, but previous experiences had shown it is rapidly recovered [2] ime [s] ime [s] Fig. 9. urbine response for a load step on island operation: voltage (continuous line) and current (dashed line) for a load step of 22 and 38.5kW Network interconnection Distributed generation sources can be developed with different functions depending on their relationship with the distribution grid and on the load they serve. hey can be basically grouped according to the possibility of working in parallel with the main grid and to functional objectives as: units dedicated to the grid, units feeding a standalone grid, units feeding a user load usually connected to the mains, units supplying a user load and a grid with local loads. o make the units capable of correctly operating in such conditions, different control solutions can be adopted [3]. In this plant the operator must preset whether the plant must work in parallel to the mains or supply a separate grid. he automatic switch from one mode to the other one is not enabled. hus, when the grid trips, the inverter rapidly shuts down. his avoids undesired islands [4, 5]. In fact, during the parallel operation, the inverter is controlled through a closed loop of the current which amplitude and phase with respect to the grid voltage are fixed in order to give a preset power flow. During the island operation, a voltage control is adopted to ensure constant amplitude and frequency of the terminal voltage. When running in the parallel mode, the loss of the grid reference rapidly causes voltage amplitude and phase to go outside the allowed range, and thus the inverter to shut down. When running in the island mode, the inverter cannot be paralleled to the grid because the unavoidable difference between the grid and the inverter frequencies would rapidly cause a too large power flow [6]. Loss of mains response. ests have been carried out having the aim of verifying that, in all the possible operating conditions, the turbine doesn t feed back a portion of the grid after a grid trip. A special attention must be posed when the load amount which remains connected to the turbine is very close to the value the turbine was supplying before the grid trip. hree cases have been analysed in which the local load is rated 22kW and the turbine control is set to 2, 25 and 3kW. In these three cases the inverter has shut down within 1 second. Frequency [Hz] ime [s] Fig.. Grid frequency following the grid trip. Further tests are planned to verify the control behaviour during voltage sags, that is, transients in which the grid voltage is largely reduced but not down to zero. CIRED23_AVpaper_UPI_Barsali.doc Academic Village Paper No - 4 -

5 Automatic reclosure. hrough a fast open-close sequence (25-3ms), it has been checked that the inverter control can resynchronise with the grid even if, during the separation, a large phase shift has been reached (see fig. 11). his test enables us to hypothesise that the inverter interfaced devices can withstand strong grid disturbances without needing to shut down, but rapidly resuming the normal operation [3]. Obviously, a suitable inverter control must be designed to pass the separation time without causing an improper shut down. CONCLUSIONS he experimental plant installed shows the feasibility of a cogeneration plant based on a microturbine and an absorption chiller which makes exploitable the residual thermal energy of the turbine both for heating and chilling purposes. he first measurements and the overall energy balance show a low electric efficiency of the non-recuperated microturbine and the need for heat insulation in the high temperature sections of the exhaust duct. A large power amount is lost there and the absorption chiller inlet temperature is C lower than the design value. he electrical tests show a fast response of the electronic interface but it appears to be rather sensitive to grid disturbances. he automatic switch from the parallel to the island operating mode is not enabled, and each grid trip forces the unit to shut down. Further tests are planned to check the immunity level with respect to voltage sags and short interruptions ime [s] ime [s] ime [s] Fig. 11. hree phase voltages (continuous line) and currents (dashed line) at reclosing time REFERENCES [1] S.Banetta, R.Giglioli, F.Paganucci, 21, Set-up and testing of a combined heat and power (CHP) plant composed by a micro gas turbine and an absorption chiller/heater ASME urbo Expo 21, New Orleans, LA, USA, June 4-7 [2] S.Barsali, M.Ceraolo, R.Giglioli, P.Pelacchi, 1999, Microturbines for dispersed generation, CIRED 1999, Nice, France, June 1-4 [3] S.Barsali, D.Poli, 23, Innovative techniques to control inverter interfaced distributed generation, CIRED 23, Barcelona, Spain, May [4] S.R.Wall, 21, Performance of inverter interfaced distributed generation, IEEE PES ransmission and Distribution Conference and Exposition, Atlanta, GA, USA, October 28-November 2 [5] R.H.Lasseter, 22, MicroGrids IEEE-PES Winter Meeting, New York, NY, USA, January [6] S. Barsali, M.Ceraolo, P.Pelacchi, D.Poli, 22, Control techniques of dispersed generators to improve the continuity of electricity supply, IEEE PES Winter Meeting, New York, NY, USA, January [7] S.Banetta, S.Barsali, M.Ceraolo, R.Giglioli, P.Bolognesi, 22, Microturbine cogeneration units, UE: Power technology, Vol. 3, N 4, July CIRED23_AVpaper_UPI_Barsali.doc Academic Village Paper No - 5 -