1、Prof. Dr. Marija Todorovi, Lic. Mech. Eng. University of Belgrade; Southeast University*, Nanjing and VEA-INVI Ltd. Belgrade, Serbia. M.Sc. Duan Liina, Mech. Eng. University of Belgrade, Belgrade, Serbia. Parametric Analysis and Thermodynamic Limits of Solar Assisted Geothermal Co- And Tri-Generatio
2、n Systems Marija S. Todorovi, PhD, PE Duan . Liina, M.Sc Fellow ASHRAE ABSTRACT This paper presents a study on the technical feasibility of efficient/cost-effective use of relatively low temperature geothermal waters for co- and tri-generation of electricity and heat for heating and/or cooling by ab
3、sorption refrigeration for building integration. As a result of global warming a need for cooling, particularly air-conditioning of buildings is in extreme growth. In Central and Southeastern Europe, as well as in many other regions in the world rich in low temperature geothermal waters 1000C (2120F
4、), there is a growing interest of governmental, public and private investors in funding the construction of energy plants which could utilize these waters in an efficient and cost effective way. In addition, current irreversibility of fossil energies/environmental exhaustion increases the importance
5、 of R the third well re-injects filtered water. The geothermal capacity of the water produces enough thermal energy for the activities of the water features, the full build out of the resort community and much more. However, in addition to the different purposes related heating demands, which can be
6、 satisfied by the available geothermal energy supply, the Aqua Tethys Spa community has significant electricity demand for cooling, air-conditioning (AC), lighting, and operation of many other technical systems. To satisfy both - heating and AC needs, available geo-sources thermal energy of enough h
7、igh temperature can be by the CHP or co-generation converted to electricity and heat. Heat can be used for heating in winter and alternatively in summer for cooling via absorption refrigeration system performing in that case the so-called tri-generation. Next example, in Serbia more than 60 hydro-ge
8、othermal low-temperature systems, below and about 1000C (2120F), present a large potential (highest temperature levels in broader region - ranked among the hottest in Europe). Estimated energy reserves of these geo-resources are about 800 MWt. Currently, in Vranjska SPA with the highest temperature
9、levels, about 1000C (2120F), a DH - district heating (including sanitary water and swimming pools) and AC is planned implementing absorption refrigerating systems in some of the DH substations and vapour compression refrigerating units powered by the gridelectrical energy in other (during heating se
10、ason these units will be also used for heating in their heat-pump operational regime). Similar examples as these in Central and Southeastern Europe, can be found in many other regions in the world rich in low temperature geothermal waters 1000C (2120F). At the same time, there is a growing interest
11、of governmental, public and private investors worldwide in funding the construction of energy plants which could utilize these waters in a more efficient and cost effective way than it is practice today. Hence, it is necessary to explore technical feasibility of efficient/cost-effective use of these
12、 waters for co- and tri-generation of electricity and heat for heating and/or cooling by absorption refrigeration. Investigation is necessary to identify the most cost-effective configuration to harvest low temperature geothermal energy for co-generation and tri-generation systems assisted by solar
13、energy or some other locally available renewable energy source such as biomass /8/. Technical feasibility, efficiency, and cost are to be explored using low temperature geothermal fluids for co-generation systems to produce electricity and thermal energy for heating, and/or for tri-generation produc
14、ing electricity, heating and cooling via absorption refrigeration processes. Relevant studies of building thermal and electrical load dynamics, and corresponding demands, should be performed based on optimum co-generation systems. It is well known that the Kalina thermodynamic cycle can convert rela
15、tively low temperature energy, at relatively low temperature compared to the heat sink or ambient temperature, to mechanical power and further to electricity. The Kalina cycle has a potential for significantly higher exergy efficiency compared to conventional Rankine cycle because, unlike pure fluid
16、s, the ammonia-water mixture has variable boiling temperature. There are also some other thermodynamic cycles and processes of interest which could be potentially used for utilization of geothermal fluids at even lower temperatures than those required for the pure Kalina cycle. In addition, there is
17、 possibility of hybridization integration of the use of low temperature geo-waters and solar or other RES to increase the geothermal fluid temperature upstream of CHP systems /8/. Namely, it is generally assumed that if the resource temperature is higher than about 90C (194F), it can be utilized to
18、generate electricity. However, it is nearly impossible to get any offer at the market, even from those producers who affirm that they are designing and engineering the utilization of hydrothermal resources with temperatures about 100C (212F). This paper carries further the study /8/ with an aim to e
19、xplore technical possibilities to expand the low-temperature Kalina cycles geo-water utilization for co- and tri-generation based exactly on the co-utilization/ hybridization of geothermal with solar or other RES. Conducted is parametric analysis and determined are relevant thermodynamic limits of c
20、orresponding systems, which encompass relevant parameters including the cooling source and local site climate conditions, beside the HVAC and other energy loads demands. 2011 ASHRAE 23Short review of the previous similar studies shows that “Combined Cycle and Waste Heat Recovery Power Systems Based
21、on a Novel Thermodynamic Energy Cycle Utilizing Low-Temperature Heat for Power Generation” had been for the first time presented by Kalina at the 1983 Joint Power Generation Conference, Indianapolis, Indiana /1/. Rodgakis and Antonopoulos /2/ /3/ analyzed a Kalina power cycle driven by a heat source
22、 of high and moderate temperatures operational with three pressure levels. In this cycle the heat contained in the exhaust steam is used to drive a “thermal compressor” allowing a higher turbine expansion ratio and a higher efficiency. Kalina and Leibowitz /4/ presented a power cycle for geothermal
23、applications showing that the Kalina cycle has a higher power output for a specified geothermal heat source compared with organic Rankine cycles and steam flash cycles. P. A. Losos and E. D. Rogdakis /7/ performed the thermodynamic analysis of a dual pressure Kalina power cycle operational at the lo
24、w temperature heat sources (similar to the power unit installed in Husavic-Iceland /5/). They presented an improved configuration which appears to have a better performance. In addition, in their study /7/ thermodynamic analysis of the cycle is conducted and the equation set has been developed which
25、 correlates the operational parameters with the theoretically achieved efficiency. More recently, R.Senthil Murugan and P. M. Subbarao presented thermodynamic analysis of Rankine-Kalina combined cycle in /11/ and Na Zhang and Noam Lior did analyze use of low and mid-temperature solar heat for thermo
26、chemical upgrading of energy, with application to a novel chemically-recuperated gas-turbine power generation system (SOLRGT) in /12/. This paper presents analysis of thermodynamic limits of a new concept of boosting the relatively low temperature geo-water sources using solar or other locally avail
27、able renewable energy sources to enable energy efficient co-generation and tri-generation by increasing the level of “high” temperature turbine inlet, and getting enough high temperatures of co-generated heat for its efficient use for the heating and/or absorption cooling purposes. In addition to th
28、e introduction of the concept of co-utilization/hybridization of geothermal with solar or other RES, this paper presents an extension of the study /7/ and continuation of the study /8/. DESCRIPTION OF THE CYCLE The Kalina cycle uses a working fluid comprised of at least two different components (typ
29、ically water and ammonia). The ratio between those components is varied in different parts of the system to increase thermodynamic reversibility and overall thermodynamic efficiency. There are numerous variants of Kalina cycle systems specifically applicable for different types of heat sources. Sinc
30、e the phase change from liquid to steam is not at a constant temperature, the temperature profiles of the hot and cold fluids in a heat exchangers can be made closer, thus making the overall efficiency of the heat transfer higher. Several proofs of concept power plants using the Kalina cycle have be
31、en built. On Fig. 1 /(in /7/ fig.2) is shown a simplified scheme of the co-generation, alternatively tri-generation unit arrangement installed in Husavic-Iceland based on a Kalina cycle. A brief description /7/ of the cycle follows. By the co-utilization of geothermal energy (from the production wel
32、l) and solar energy (from the solar collector field) necessary thermal energy is supplied to the cycles working fluid in evaporator. After releasing heat in evaporator, geo-water and the solar collector fields working fluid are used for heating purposes: lower exergy geo-water for SPA and agricultur
33、e, and higher exergy value solar collector field water for district heating system (DHS) and/or district cooling system (DCS). Characteristic states of the Kalina cycle working fluid are as follows. Starting at the outlet of the absorber (State 0) the strong solution of a mass fraction Xris saturate
34、d at a low pressure pL. This stream is pumped to a high pressure by the feed pump (State 1r). The feed stream is preheated in the low temperature (State 2r) and the high temperature (State 3r) recuperators before entering the evaporator. In the evaporator the mixture is heated by the heat source (I
35、step geothermal source and II step solar or biomass) to TH, where it is partially vaporized (State 4r). The mixed-phase fluid is sent to the separator where the basic solution is separated into an enriched vapor (Xv) (State 5) and a weak liquid solution Xw(State 4w). The high-pressure, strong satura
36、ted vapor from the separator drives the turbine as the vapor expands and cools to a low temperature, low pressure exhaust (State 7). The saturated liquid solution (State 4w), after recuperating some of the heat at the high temperature recuperator, is throttled down to a low pressure (State 2w). Then
37、 the expanded stream (state 7) mixes with the weak stream (State 2w), condenses and forms the basic solution completing the cycle (this is achieved through a counter-current absorber). The connections for the absorber heat rejection have been omitted in Fig.1 due to simplicity. There is though a coo
38、ling circuit system which works as follows: firstly the cooling fluid absorbs the heat rejected from the absorption process (change 8r-0r or 2w-0r); then, the absorbed heat, through the cooling circuit, is used to preheat the rich solution (change 1r-2r). In the same paper /7/ had been analyzed also
39、 the same power generation unit arrangement, but with co-current absorber. As the study /7/ did show important advantage of obtainable cycles higher thermal efficiency, in this study such configuration has been selected as an initial reference for the further parametric thermodynamic analysis. 24 AS
40、HRAE TransactionsThe co- and tri-generation modes of this type of Kalina based power generation unit assumes utilization of the residual thermal potential of geo-fluid after releasing heat in the first part of the evaporator section for the lowest level heating purposes, and also utilization of the
41、residual thermal potential of solar heated working fluid after the evaporators second section (for heating and when necessary or absorption cooling tri-generation), as well as further utilization of the condensation heat transferred to the cooling fluid/environment depending on its temperatures. Fig
42、. 1 Schematic arrangement of a Kalina power cycle with a counter-current absorber An ideal Kalina cycles relevant independent variables are: cycle low pressure, pL; cycle minimum temperature TL(temperature at condenser exit); cycle maximum temperature; TH(temperature at boiler exit). Streams involve
43、d in the cycle are: a weak solution of mass mw=g kg and mass fraction Xw(change 2w-4w); a strong solution of mass mr=(1+g) kg and mass fraction Xr(change 1r-4r); and a rich vapor stream of mass mv=1 kg and mass fraction Xr(change 5-7). Cycle modeling has been performed /7/ using the following assump
44、tions: x All processes within the cycle, excluding pumps, throttling valves and the turbine, are considered as constant pressure processes. x The enthalpy increase in pump is assumed to be enough small to be neglected. x The strong mixture at condenser exit (State 0) and at the evaporator inlet (Sta
45、te 3r) are at saturated condition. x The weak mixture after its throttling to the low pressure (state 2w) is at saturated condition. x Heat losses in the piping and the heat exchangers are enough small to be neglected. Thermodynamic properties of the Kalina working fluid - mixture of NH3-H2O are cal
46、culated /7/ using the equations of Ziegler and Trepp /6/. In this study, thermodynamic analysis of the ideal Kalina cycle defined in /7/ has been extended using the variables and assumptions listed above, and using the mass balance and a set of relevant equations to calculate the boiler heat transfe
47、r, absorber heat rejection, work output and thermal efficiency (for 1 kg of vapour expanded in the turbine) as follows: x Boiler heat transfer 52inqhh (1) x Absorber heat rejection 72 absqhh (2) x Work output 57 twhh(3) 2011 ASHRAE 25x Thermal efficiency tinwqK (4) Boiler heat defined by the equatio
48、n (1), is given as a whole and not reduced for the recuperated heat within the cycle itself. As this new concept of increasing the heat source temperature by the composition of geo and other RES aims to reach a CHP (co-generation) efficiency high enough for practical cost-effective implementation ev
49、en with the available low temperature geo-sources, the main issue is analysis of the impact of increased temperature of heat source (adding solar to geo), and analysis of different cooling fluid/environment temperatures on thermodynamic limits. Thus, the heat recovery within the cycle has been treated in the same way as in the referential study /7/. However, recuperators role and especially its heat transfer efficiency has additional positive impact on the Kalina cycles efficiency improvement. . PERFOROMANCE DATA O
