GEOTHERMAL BINARY PLANTS: WATER OR AIR COOLED? D.Mendrinos, E.Kontoleontos and C.Karytsas

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1 GEOTHERMAL BINARY PLANTS: WATER OR AIR COOLED? D.Mendrins, E.Kntlents and C.Karytsas Centre fr Renewable Energy Surces 19 th km Marathns ave., Pikermi Attikis, Greece Abstract Cling gethermal pwer plants is necessary in rder t cndense the vapur feeding the turbine, lwer the heat rejectin temperature, raise pwer utput and increase heat t pwer cnversin efficiency. Three main cling ptins are used: a) surface water (nce-thrugh systems), b) wet type cling twers, and c) dry type cling twers. Cling with surface water yields the lwest cndensing pressure and temperature and the highest cnversin efficiency, fllwed by wet cling twers, and then by dry cling twers. Regarding the need fr cld water supply, the rder is reversed. Typical values are 970 t/h, 30 t/h and zer t/h respectively per MWe f installed pwer. In terms f csts, nce thrugh cling may require bth high capital csts and electricity cnsumptin fr transprting water. Dry cling is the mst expensive ptin due t the much higher heat capacity and heat transfer cefficient f water cmpared with ambient air. A dry cling twer fr a binary pwer plant f high cnversin efficiency may cst 10 times mre than its wet cunterpart, which may result in raising verall pwer plant csts by 50%. In flash plants, where there is plenty f steam cndensate t use as make up water, the standard technlgy adpted almst exclusively is cst effective direct cntact cndensers cupled with wet cling twers. In binary plants, where the mre expensive shelland-tube r plate heat exchangers are used as surface cndensers, the selectin f the cling system type is gverned by water availability, lcal water use regulatins and ecnmics. 1. Intrductin The gal f this paper is t examine and cmpare the different cling ptins adpted in gethermal pwer plants. Fr ur analysis we divide gethermal pwer plants int the fllwing main types: Back pressure flash plants where the steam is discharged frm the turbine at 1 bar(a) pressure r 100 C. Cndensing flash plants which use a cndenser in rder t cndense the steam at the turbine discharge at lwer pressure and temperature. Binary plants, which use the gethermal ht water r steam t bil a clsed lp f a secndary wrking fluid, which drives a turbine and is cndensed at the turbine discharge, and then is cnveyed by a pump t the gethermal surce and fllws the same cycle again and again. Present technlgy fr cling the pwer plant cndensers may use either r bth water r air and includes: 1

2 Cling with surface water in nce thrugh systems Cling with water evapratin in air draft by wet type cling twers Cling with air by dry type cling twers 2. Need fr cling The Carnt efficiency f a gethermal pwer plant, either flash r binary, is: Tg Tc n = (1) Tg where: Tg is the temperature f the gethermal surce in degrees Kelvin, and "Tc " is the cling water temperature in degrees Kelvin Given the temperature f the gethermal surce Tg, frm equatin (1) it is evident that the Carnt efficiency f the plant is increased if we decrease Tc, which can be achieved by cndensing the steam r fluid vapur t the lwest pssible temperature. This can be achieved by intrducing a cndenser at the turbine discharge, which shuld discharge the latent heat f the cndensate t a cling fluid. If we cnsider the turbine alne, fr a given mass flw rate and specific enthalpy f the gethermal steam the delivered mechanical wrk t the pwer generatr w equals t: where: t ( H H ) w = n m (2) s m is the steam mass flw rate, H s is the specific enthalpy f the steam r fluid vapur entering the turbine, H is the specific enthalpy f the steam r fluid vapur at the turbine discharge, and n t is the turbine efficiency, usually within the range 60%-90% The pwer N generated at the generatr equals t: where: N = n w (3) g n g is the efficiency f the generatr, usually arund 95% Overall heat t pwer cnversin efficiency f the plant n equals t: s ( H H ) N ng nt s n = = (4) m H H s 2

3 Frm abve equatins (2), (3) and (4) it becmes evident that in rder t increase the pwer generated by a given quantity f gethermal steam r wrking fluid vapur, and hence the heat t pwer cnversin efficiency, we shuld minimize H.This can be achieved by discharging the steam at the turbine exit t the minimum pssible temperature, which, accrding t the thermdynamic prperties f the fluids, crrespnds t the lwest pssible pressure. Such lw pressure can be generated by cndensing the steam r fluid vapur t the lwest pssible temperature with a cling fluid. We arrived therefre t the same cnclusin with the Carnt efficiency analysis f equatin (1). 94% f the ttal generated pwer glbally is derived by steam flash plants, the majrity f which use cndensing turbines, which in practice yield twice as much pwer utput than the atmspheric exhaust nes. Due t the thermdynamics invlved, all gethermal binary plants, which crrespnd t 6% f glbal gethermal pwer utput, use wrking fluid cndensing turbines. 3. Water vs. air cled cndensers In nature, available cling fluids are either water (frm sea, lakes, rivers, r subsurface) r air. In terms f heat transfer, water has mre favurable prperties than air, as fllws: Water has ver 4 times higher specific heat c = 4,19 kj kg C than ambient air c = 1,00kJ kg C. pa / pw / Water is 830 times mre dense than air. Fr example fr cling fluid temperature f 15 C, the water s density is 999 kg/m³ whereas the air s density is 1,2 kg/m³. This results in higher vlumetric heat capacity and heat transfer cefficient. Water has vlumetric heat capacity VHC w 3 = 4182kJ / m C, apprximately times the ne f ambient air, which is VHCa = 1,21kJ / m C. This implies that in rder t have the same heat transfer effect, 3450 mre vlume f air has t be mved than in the case f water, resulting in the need fr bulky and expensive equipment fr air-handling, plus higher electricity cnsumptin fr the air fans than the water pumps. In cndensers water yields typical heat transfer cefficient 58 times higher 2 than the ne f air, which we have estimated as h = 4,84kW / m C fr water 2 and ha = 0,084kW / m C fr air. This implies that the surface f the cndenser and the crrespnding csts will be accrdingly higher if air is used as cling fluid rather than water. The heat exchange surface has a direct impact n the weight and size f the cndenser, which are the mst imprtant ecnmic variables defining the crrespnding csts. w 3

4 4. Surface water (nce-thrugh systems) In this categry the cling fluid is water, which is transprted t the pwer plant thrugh pipes frm a river, a lake r the sea. The temperature f the cling water in this case varies in prprtin t seasn s temperature. It can be 5 C - 25 C. This is why surface water yields the lwest cndensing pressure and temperature in cmpare t the ther tw types. A typical value f water supply is 970 t per h and per MWe f installed pwer fr apprximately 10 C temperature gain f the cling water acrss the cndenser. As far as it cncerns the plant s cst, electricity cnsumptin fr transprting water (pipes, pumps etc.) may nt be at all negligible, depending n the lcatin and distance f the water surce. Binary plants nrmally use hrizntal duble pass shell-and-tube heat exchangers as surface cndensers, with the cling water flwing inside the tubes and the steam and cndensate in crss flw within the shell. Althugh nt a standard practice, use f plate heat exchangers instead, may be a tempting ptin due t their cmpact size, their mass prductin and easy t dismantle/mantle and clean capabilities and their high verall heat transfer cefficient, typical values f which are kw/m². In cases where sea, lakes r rivers are lcated clse t the pwer plant, and in cases where lcal regulatins fr water use allw it, cling with surface water shuld be cnsidered as ne f the available ptins. Its main advantage is that it can yield the lwest pssible cndensing temperature, and hence the maximum cnversin efficiency as: Surface waters tend t have lwer temperature than ambient air during the summer perid; fr example in Suth Eurpe sea water has temperature f ~25 C during the summer, while ambient temperatures arund 35 C are cmmn. In mst Eurpean cuntries surface waters d nt frze when ambient temperature drps belw 0 C. N cling twers f either wet r dry type are necessary. The heat delivered t the cling water can be utilized fr dwnstream heating applicatins, resulting in a gethermal heat and pwer cgeneratin plant and further increasing verall energy efficiency. Main drawbacks include: Need fr large water quantity. Fling r crrsin in the cndenser in cases f adverse chemistry, r rganisms present in the cling water High capital csts fr piping and pumping statins r electricity cnsumptin in case the water has t be transprted frm large distances. 5. Wet type cling twers The cling water that is used in the cndenser is cnveyed t the cling twer in rder t reduce its temperature s that it will be recycled and lped thrugh the system. An imprtant reductin f its temperature is accmplished in the twer. In small r medium size plants, such as gethermal pwer plants, cling twers usually use mechanical ventilatin (fan) fr the advectin f the air stream. In these 4

5 plants cling twers that are mstly used are crss-flw and traverse-flw. The typical temperature difference between the inlet and utlet cling water is 10 C. As far as it cncerns the temperature f the cling water that cmes ut f the cling twer, it reaches at least 25 C, resulting in cndensing temperatures arund 40 C, depending n the ambient temperature. Wet cling twers cmbine the use f water as a cling media t the cndenser and benefit frm its favurable heat capacity and heat transfer prperties cmpared with air, while they d nt need the large vlumes f surface water needed in nce thrugh cling systems. Instead, they evaprate water at a cling twer, and need a much smaller quantity f make up water t cmpensate the evapratin lsses plus the water blwdwn necessary t maintain water quality. Typical needs fr make up water f a gethermal binary plant have been estimated as 30 t/h per MWe f installed pwer capacity. In gethermal flash plants, there are usually enugh water quantities available fr the make up water f the cling twers, as the much less make up water needed per MWe f delivered pwer, crrespnds t a fractin nly f the available steam cndensate. There, wet cling twers are usually cupled with direct cntact cndensers, where the cling water is sprayed and mixed with the steam cndensate, and which are simpler in design and much mre cst effective than surface cndensers used in binary plants. Fr this reasn, direct cntact cndensers and wet type cling twers are the standard technlgy in gethermal flash plants. Exceptins are encuntered in cases where large quantities f surface water are available lcally, and in extremely cld climates in rder t avid frsting water drplets precipitating in the plant neighburhd. 6. Dry type cling twers In dry type cling twers, the temperature f the air that cmes ut f the twer in rder t cl the fluid in the cndenser is higher than 25 C. Typical values are C resulting in cndensing temperatures arund C. In a dry type cling twer n water supply is necessary. Regarding auxiliary pwer cnsumptin, they usually cnsume twice as much electricity than wet cling twers. Due t the need fr many times higher heat exchange surface and the large vlume f air that has t be mved thrugh them, dry type cling twers are the mst expensive ptin. A dry type cling twer csts 5-10 times as much as a wet type ne depending n the cndensing temperature f the turbine. If lw cndensing temperatures are cnsidered, an air surce gethermal binary plant may have 50% higher capital csts than ne with a wet type cling twer f the same efficiency. In practice, air surce gethermal binary plants are designed with cnsiderably less cnversin efficiency and csts 10-20% higher. Hwever, in cases f lack f water, strict lcal water use regulatins, extremely lw ambient temperatures during winter which cause water drplets frm wet type cling twers t freeze nt nearby vegetatin, dry type cling twers may be the nly available ptin. 5

6 7. Rankine cycle ptimisatin In the framewrk f the LOW-BIN (Efficient Lw Temperature Gethermal Binary Pwer) prject, which is c-financed by the 6th Eurpean framewrk prgramme (FP6), we have been mdelling Rankine cycles in rder t ptimise csts and cnversin efficiency. Optimisatin has been perfrmed using the EASY sftware cde (Evlutinary Algrithm System by Natinal Technical University f Athens, ref. Fr the abve ptimisatin a lw temperature binary plant has been cnsidered, supplied by 65 C gethermal water, with R134a as wrking fluid. The cling water temperature is 10 C. The plant is schematically presented in Figure 1. Figure 1: Binary plant layut. Fr the initial phase f the prject we have mdelled the Rankine cycle using certain assumptins, such as simplified gemetry fr the cndenser, which will be relaxed in later stages. The verall heat-transfer cefficient used fr mdelling the heat transfer at the cndenser is based n the fllwing frmula: U = A A i 1 h i + A 1 ln ( r / r ) 2πkL i 1 + h (5) 6

7 where A and A i represents the ut and in surface areas respectively f the inner tubes, L is the length f the tubes, h i is the heat transfer cefficient inside the tubes where the cling fluid flws, h is the heat transfer cefficient utside the tubes where the wrking fluid flws and k represents the thermal cnductivity f the tube s material. The heat transfer cefficient utside the tubes (R-134a) is based n the fllwing frmula fr laminar film cndensatin n hrizntal tubes: h ( ρ ρ ) ρ = µ f d v ( T T ) g gh fg w k 3 f 0.25 (6) where ρ and ρ v represent the density f R-134a in liquid and vapur frms respectively, h fg is the latent heat, µ f is the dynamic viscsity, k f is the thermal cnductivity, d the utside diameter f the tube, T g is the saturatin temperature f the fluid cndensate and T w the temperature f the tube s wall. The heat transfer cefficient inside the tube fr turbulent flw is based n the fllwing frmulas : Nuk h i = D Nu = 0.023Re 0.8 Pr 0.4 (7) The R134a Rankine cycle was ptimised defining the fllwing variables: the utlet pressure f the pump,p 2 the ht grund water supply, m gr the supply f the R-134a in the cycle, m R134a the temperature difference f the grund water in the heat exchanger, Τ Η the temperature difference f the cling fluid in the cndenser, Τ C The crrespnding upper and lwer limits are listed in table 1. Table 1. Upper and lwer limits f the ptimisatin variables Variable Lwer limit Upper Limit p 2 (kpa) m gr (kg/sec) m R134a (kg/sec) Τ H ( C) Τ C ( C) 7,5 12,5 7

8 The bjectives f the ptimizatin are: Maximizatin f the ttal efficiency f the plant η cycle = w q turbine heatexch = h h 4 3 h 5 h 2 Minimizatin f the cst f the plant. Since the cst f the heat exchanger and the cndenser cnstitute a majr part f the plant cst, fr ur ptimizing purpses we can substitute the plant cst by their cst. S the new gal is t minimize the cst f the heat exchanger and the cndenser which is prprtinal t their surface Minimizatin f the exchangers surface. The results are presented in figures 2 and 3 fr water cled and air cled cndensers respectively. A cmparisn f selected representative cases is shwn in table 2. In figures 2 and 3 a number f ptimal slutins have been plt. Each slutin is represented by tw numbers which cnstitute the surface f the heat exchangers (gethermal heat exchanger and R134a cndenser) and the verall cnversin efficiency. Additinally, each slutin resulted frm a different set f variables crrespnding t an ptimal Rankine cycle. In water cled systems (figure 2), the values f ttal efficiency vary between 6,1% and 7,2%, when the ttal surface f the heat exchangers is between 100 and 250 m². As we can bserve frm the diagram, after the value f 135 m², very small efficiency rises are assciated with large increments in the heat exchangers surface. Fr this reasn we cnsider a representative slutin crrespnding t heat exchangers surface and cnversin efficiency f 138 m² and 6,96% respectively. In air cled systems (figure 3), the values f ttal efficiency are between 6,0% and 6,9%, when the ttal surface f the heat exchangers is between 2400 and 4000 m 2. As abve, we can select as a representative slutin the ne that crrespnds t heat exchangers surface and cnversin efficiency f 3230 m² and 6,78% respectively. If we cmpare figures 2 and 3 and the results presented in table 2, we cnclude that the surface f a water cled cndenser is apprximately 25 times less than an air cled ne. The ttal heat transfer cefficient f a water cled cndenser is ~5600 W/m 2 C when the air cled cndenser is nly ~100 W/m 2 C. Csts fr air-cled cndensers shuld be higher than a water cled nes accrdingly. 8

9 ttal efficiency f binary cycle 2 bjectives f the ptimisatin surface f the exchangers Figure 2: Rankine cycle ptimizatin fr water cled cndensers bjectives f the ptimisatin ttal efficiency f binary cycle surface f the exchangers Figure 3: Rankine cycle ptimizatin fr air cled cndensers 9

10 Table 2. Cmparisn f representative ptimized Rankine cycles fr water and air cled cndensers. Variable Water Cled Air Cled p 2 (kpa) m gr (kg/sec) 52,3 53,0 m R134a (kg/sec) 17,5 17,5 Τ H ( C) 17,5 17,8 Τ C ( C) 7,5 7,5 R134a pump pwer (KW) cling fluid flw (m³/h) 403 3,45*10 5 Ttal heat transfer cefficient U Surface f the cndenser (m 2 ) Ttal H.E. surface (m 2 ) Cnversin efficiency 6,96 % 6,78 % 8. Cnclusins Cling gethermal pwer plants is necessary in rder t imprve cnversin efficiency. Water cling leads t higher cnversin efficiencies and lwer plant capital and peratin csts than air cling, as has been prved by analysing heat transfer prperties f water and air, as well as by ptimizing the crrespnding Rankine cycle. On the ther hand, water cling needs cnsiderable quantities f cld water supply. Use f water cled cndensers and wet type cling twers results in drastically reduced, but still cnsiderable water needs. Dry air type cling twers, despite adverse ecnmics and energy efficiency, may be the nly feasible ptin in cases f water scarcity, r extreme climatic cnditins. 10