Thermodynamic analysis of low-temperature geothermal energy sources

Transcription

1 Thermodynamic analysis of low-temperature geothermal energy sources Case-study location Acquapendente (VT) Italy Activity 4.2 in WP 4 Partner: CEV Consortium Coordinated by: MF

2 Index SUMMARY 3 1 INTRODUCTION 4 2 HYDRAULIC SCHEME OF THE HYBRID SYSTEM 5 3 TECHNICAL DATA OF THE HYBRID SYSTEM General characteristics of the system Geothermal system data 12 REFERENCES 13 1

3 List of figures Figure 1 Identification of the position of the well considered in the analisysl 3 Figure 2 Example of the main components of geothermal power plants 4 Figure 3 Example of the main components of solar power plants 4 Figure 4 Geothermal ORC system hydraulic scheme 5 Figure 5 Solar ORC system hydraulic scheme 5 Figure 6 Hybrid Solar-Geothermal ORC system hydraulic scheme 5 Figure 7 Hybrid Solar-Geothermal ORC system simplified hydraulic scheme 6 Figure 8 Rapresentation of the transformation on Pressure-Enthalpy Diagram 7 List of tables Table 1 List of wells in case-study location 3 Table 2 Main characteristics of the considered well Error! Bookmark not defined. Table 3 Thermodynamic properties of the points of the power cycle 8 2

4 Summary Case-study location has been identified in the municipality of Acquapendente. 3 Figure 1 Identification of the position of the well considered in the analysis In the area there are a number of wells for which data principlai are summarized below. Table 1 List of wells in case-study location Lat Lon Elevat. Municipality Geothermal Flow rate Temp. Type of well Field (kg/s) ( C) Acquapendente Alfina Exploration Well Acquapendente Alfina Exploration Well Acquapendente Alfina Exploration Well Acquapendente Alfina Production Well For the case-study location the main system characteristic and the main parameters assumed necessary for a subsequent verification of technical and economic feasibility have been identified and presented in the document.

5 1 Introduction The paper describes the main aspects of a hybrid geothermal and solar system proposed for the pilot case of the village of Acquapendente. Type of the system and purpose o RES used: Geothermal and Solar Figure 2 Example of the main components of geothermal power plants Figure 3 Example of the main components of solar power plants o Case study location for evaluations of the proposed system: Municipality of Acquapendente, in the Province of Viterbo in the area of geothermal interest of Alfina upland. 4 o System aim Production of electricity; it is eventually possible to exploit residual heat for a local district heating network. o Electricity uses Grid connected plant; it is assumed that the electricity is sold to the grid. o Operating mode of the hybrid system designed to work all year; annual operating hours assumed; Solar system integrate geothermal source base load in certain periods depending on the availability of solar radiation. The main characteristics of the borehole under consideration, which are listed in the table above, used for the design calculatios have been derived from the Geothermal resources national inventory published in Dicembre 1987 and updated in Update on febbraio 23, 2010.

6 2 Hydraulic scheme of the hybrid system The hybrid system involves the combined use of geothermal and solar energy. Combining example hydraulic schemes developed by the Faculty of Mechanical Engineering of Belgrade University for a geothermal source ORC system and a solar source one we have proposed a hybrid system scheme as follows. + Figure 4 Geothermal ORC system hydraulic scheme Figure 5 Solar ORC system hydraulic scheme 5 Figure 6 Hybrid Solar-Geothermal ORC system hydraulic scheme

7 3 Technical data of the hybrid system 3.1 General characteristics of the system The design of the system has been developed on the basis of the technical information contained in the article "Technical and economical analysis of a solar geothermal hybrid plant based on an Organic Rankine Cycle" by Marco Astolfi, Luca Xodo, Matteo C. Romano, Ennio Macchi from Energy Department of Politecnico di Milano. Following the reference schemes reported in the above mentioned article and on the basis of the characteristics of the hybrid system (solar-geothermal) under consideration, the following simplified hydraulic scheme has been developed: 6 Figure 7 Hybrid Solar-Geothermal ORC system simplified hydraulic scheme In figure 8 below the same system is rapresented on a Pressure-Enthalpy Diagram assuming that the working fluid is Refrigerant 245fa.

8 Figure 8 Rapresentation of the transformation on Pressure-Enthalpy Diagram 7

9 In the Table 3 below the main thermodynamic properties of the points of the power cycle of thr hybrid geothermal and solar system are shown. Table 2 Thermodynamic properties of the points of the power cycle Point G (kg/s) T ( C) h (kj/kg) 1 114,596-32, , ,596-32, , ,596-32, , , , , , , , ,596 30, , ,596 30, ,000 This working fluid has been chosen after considering the characteristics of the cycle that will be used to produce electrical energy. For each step the input data and calculations done are explained below: Point 1: Starting point The starting point has been established at the following values of temperature and enthalpy (1) T 1 = - 32 C (2) h 1 = 157 kj/kg 8 Point 1 Point 2: Pump From point 1 to point 2 the working fluid is compressd by the circulation pump with an increase of the pressure but with temperature and enthalpy remaining constant. (3) T 2 = T 1 (4) h 2 = h 1 Point 2 Point 3: Regenerator In this early stage, we have not considered the use of a recuperator aimed at pre-heating the working fluid entering the primary heat exchanger via a heat exchange with the fluid leaving the turbine before it reaches the condenser. Therefore, excluding the use of a recuperator, we have: (5) T 3 = T 2 (6) h 3 = h 2

10 Point 3 Point 4: Primary Heat Exchanger From point 3 to point 4, the working fluid passes through the primary heat exchanger and acquires heat from the geothermal fluid; To calculate the heat transferred from the geothermal fluid we use the main features of the well being considered which are reported in the previous tables 1 and 2, in particular the values of the geothermal fluid entering the heat exchanger (point a) and the fluid leaving the heat exchanger (point b) are (7) G a = 139 kg/s (8) T a = 110 C We envisage a minimum injection temperature of 70 C, therefore (9) G b = 139 kg/s (10) T b = 70 C and consider the following values of enthalpy and specific enthalpy for the geothermal fluid: (11) h geo = 481,16 kj/kg (12) s geo = 4,184 kj/(kg K) We then calculate the power transferred from the geothermal fluid from point a to point b: 9 (13) Q geo a-b = G geo x s geo x (T a T b ) = 139 [kg/s] x 4,184 [kj/(kg K)] x (110 70) [ C] = Q geo a-b = kw If we assume a 100% efficiency for the heat exchanger, this power is fully transferred from the geothermal fluid to the working fluid: (14) ƞ heat,exch = 100% (15) Q work 3-4 = ƞ heat,exch x Q geo a-b = Q geo a-b = kw Therefore, using the values of the working fluid in point 4: (16) T 4 = T a = 110 C (17) h 4 = 360 kj/kg (obtained from the pressure-enthalpy diagram of the working fluid) it is possible to calculate the mass flow of the working fluid in the circuit (assumed to be constant): (18) G work = Q work 3-4 / (h 4 h 3 ) = [kw] / (110 (-32)) [ C] = 114,596 kg/s

11 Point 4 Point 5: Additional Heat Exchanger After the primary heat exchanger in which the working fluid exchanges heat with the geothermal fluid, our hybrid system uses an additional heat exchanger that further increases the temperature of the working fluid through the transfer of heat with a superheated fluid inside a solar system; In our assumption we design the solar system including a heat exchange with the primary circuit to reach a temperature for the working fluid in point 5 equal to 130 C, therefore: (19) T 5 = 130 C (20) h 5 = 510 kj/kg (obtained from the pressure-enthalpy diagram of the working fluid) Using these values, we can calculate the power that the primary working fluid must receive from the additional heat exchanger: (21) Q 4-5 = G work x (h 5 h 4 ) = 114,596 [kg/s] x ( ) [kj/kg] = kw If we still assume in this phase an efficiency for the heat exchanger of 100%, the power transferred to the working fluid is equal to that of the solar fluid: (22) ƞ heat,exch = 100% (23) Q sol c-d = ƞ heat,exch x Q 4-5 = Q 4-5 = kw 10 from the "Technical and economical analysis of a solar geothermal hybrid plant based on an Organic Rankine Cycle" by Marco Astolfi, Luca Xodo, Matteo C. Romano, Ennio Macchi from Energy Department of Politecnico di Milano, we obtain the formula for the relationship between the produced power and the area covered by the parabolic trough solar collector: (24) Q sol = S sol x DNI x ƞ sol where S sol = Net collecting surface DNI = Direct Normal Irradiance = 1,33 MWh/(m² year) (value for the city of Pisa) ƞ sol = solar system efficiency = 52,44% from which we obtain the net solar collecting surface necessary to raise the temperature of the working fluid to 130 C after the additional heat exchanger: (25) S sol = Q sol / (DNI x ƞ sol ) = [kw] / (1,33 [MWh/(m² year)] x 52,44%) = S sol = m²

12 Point 5 Point 6: Turbine From point 50 to point 6 the fluid goes through the turbine and transfers some of its energy to produce electrical energy: We assume an efficiency for the turbine equal to: (26) ƞ turb = 80% In an ideal transformation s remains constant, therefore: (27) s 6' = s 5 = 1,875 kj/(kg K) From which we obtain, according to an ideal transformation: (28) h 6' = 410 kj/kg (obtained from the pressure-enthalpy diagram of the working fluid The enthalpy of point 6 is therefore equal to (29) h 6 = h 5 (ƞ turb x (h 5 - h 6' )) = 510 [kj/kg] (80% x ( ) [kj/kg] ) = h 6 = 430 kj/kg (30) T 6 = 30 C (obtained from the pressure-enthalpy diagram of the working fluid) 11 So we can now calculate the power produced by the turbine: (31) Q turb = G work x (h 5 h 6 ) = 114,596 [kg/s] x ( ) [kj/kg] = kw Point 6 Point 7: Regenerator In this early stage, we have not considered the use of a recuperator aimed at pre-heating the working fluid entering the primary heat exchanger via a heat exchange with the fluid leaving the turbine before it reaches the condenser. Therefore, excluding the use of a recuperator, we have: (32) T 7 = T 6 = 30 C (33) h 7 = h 6 = 430 kj/kg Point 7 Point 1: Condenser In the last step of the energy cycle a condenser brings the working fluid from the conditions of point 7 to the initial point 1 so that the cycle can start again, so it is possible to calculate the required power for the condenser as follows: (34) Q cond = G work x (h 7 h 1 ) = 114,596 [kg/s] x ( ) [kj/kg] = kw

13 3.2 Geothermal system data a) Geothermal brine: I. Flow rate 85 [kg/s] II. Temperature 100 [ C] III. Minimum injection temperature 70 [ C] IV. Injection pump Δp 100 [kpa] V. Injection pump efficiency η pump 75% 12

14 References [1] M. Astolfi, L. Xodo, M. C. Romano, E. Macchi, Technical and economic analysis of a solar-geothermal hybrid plant based on an Organic Rankine Cycle, Geothermics 40, 2010, [2] L. Böszörményi, G. Böszörményi, Hybrid energy technologies for an efficient geothermal heat utilization,european Geothermal Conference, Retrieved from [3] Italian Ministry of Economic Development, Geothermal resources national inventory, 13