Experimental Performance Evaluation of a Small Scale ORC Power Unit Working with Low Temperature Energy Sources for Power Cogeneration

Transcription

1 Experimental Performance Evaluation of a Small Scale ORC Power Unit Working with Low Temperature Energy Sources for Power Cogeneration Maria K. Koukou, Speaker mkoukou@teiste.gr, m_koukou@otenet.gr and Michail Gr. Vrachopoulos, George Dogkas, and Nikolaos Tachos Energy and Environmental Research Laboratory Department of Mechanical Engineering Technological Education Institute of Sterea Ellada

2 RES and ORC Renewable Energy Sources (RES), such as solar thermal and geothermal energy are energy sources that can meet electricity demand. However, the moderate temperature heat from these sources cannot be converted efficiently to electrical power when using conventional power generation methods. For the conversion of lowgrade heat sources into electricity various cycles have been proposed as the Organic Rankine Cycle (ORC). The ORC applies the principle of Rankine cycle steam but uses an organic fluid with a low boiling point to recover the heat from low temperature heat sources.

3 The Effi Low Res ORC An ORC unit (Effi Low Res ORC unit) of nominal power 10kWe (electricity production) was designed and developed and it is able to operate under conditions providing by a solar thermal system or a geothermal system. The RES conditions were simulated using a boiler which by performing adequate adjustments was able to produce the final heat by solar or geothermal source in the same pace, size and a way and which accurately reflects the operation of the solar or geothermal system.

4 Operational approach The hot fluid is used to transfer part of the its heat to a secondary circuit, through a heat exchanger, the evaporator. The vapor of the secondary fluid, in particular the cooling fluid R134a, propels the turbine, which is connected with a generator and produces electrical energy. Vapor of R134a is cooled and condensed inside the condenser and then it is send to the evaporator with the use of a cooling fluid pump.

5 Design Process q Study of ORC power generation and cooling cycles for efficient low temperature ORC Power Generation combined with solar or geothermal energy; q Assessment of working fluids for the desired operating conditions; q Technology assessment based on market available equipment; q Development and laboratory testing of the ORC system.

6 Working fluid characteristics Thermodynamic efficiency Efficiency and power output should be the maximum for given temperature conditions. This is usually associated with low energy consumption pump and high critical point. Positive or isentropic vapor saturation curve Negative vapor saturation curve leads to the formation of drops at the end of the expansion. The steam must be overheated at the inlet of the expander in order to avoid its destruction and reducing cycle efficiency. High vapor density High vapor density is important, especially for fluids showing low condensing pressure. Low density leads to large equipment in the levels of expansion and condensation.

7 Working fluid characteristics Low boiling point High stability temperature Based on the Effi Low Res process requirements. Unlike water, organic fluids usually suffer from chemical wear and decays at high temperatures. Therefore, the maximum temperature is limited by the chemical stability of the fluid. Low environmental impact and high level of security Market availability and low cost

8 Working fluid Selection Refrigerants Studied q R134a, R245fa και HFE7000 were studied. q R245fa was discarded due to the high heat required amount although it has the best efficiency. q HFE7000 was rejected because of the higher required mass flow. Selected R134a as a working fluid is an optimal solution based on the balance of various factors studied e.g. required amount of heat mass flow rate market availability and cost R134a also presents a high enough yield at about 75 C and as the research work involves electricity production from solar and geothermal energy at 80 o C, it was preferred.

9 Effi Low Res ORC components The main ORC system including all devices and components through which the refrigeration cycle is implemented (pump, evaporator, scroll expander, condenser, valves, collector, measuring devices, etc.). The cooling / rejection system comprising cooling tower, piping, the water pump and accessories. Typically the ORC condenser is also included. This is one of the most essential subsystems, as firstly, it determines the low operating temperature of the organic cycle and because of a substantial part of the system self-consumption. The heating system comprising the boiler used to simulate the conditions that correspond to the solar thermal system or the geothermal system (pipes, water pump, automation, and measurement accessories).

10 Effi Low Res ORC components Scroll Expanders and Generators o o o Centrifugal expansion devices for electric power generation are not suitable for use with low boiling point fluids as R134a and for low mass flow rates. In Effi Low Res device two scroll compressors were used, connected in parallel, which were transformed into reversing operating cycle expanding devices and rated power each around 5kWel. To perform this operation, the one-way check valve was removed from their shell. o Commercial generators, three-phase 2-pole, selfactuated and with automatic stabilization system were selected.

11 Effi Low Res ORC components Heat Exchangers Heat exchanger Evaporator Condenser Type Plate Plate counterflow counterflow Thermal power 200 kw 200kW Hot side fluid Water R134a Inlet/Outlet temperature of hot side 80/ 60 o C 35 o Cgas/ 35 o Cliq Cold side fluid R134a Water Inlet/Outlet 35 temperature of o C liq/ 75 o C 27/30 o C gas cold side

12 Effi Low Res ORC components Working Fluid Pump Pressure regulation The refrigerant pump for R134a is a multi-stage commercial one, magnetically driven, rated at 4.69m 3 /h. Α refrigerant receiver was used to achieve the liquid state at the pump inlet.

13 Effi Low Res ORC components Cooling Tower For the necessary heat rejection of the device from the condenser-exchanger an open type evaporative cooling tower was used. This arrangement creates for each season the corresponding heat rejection temperature from the condenser to the ambience. The installation consists of the cooling tower, the condenser cooling water flow pump and the electrical connection. It connects to the water network to fill water, due to exhaust during operation. Cooling Water flow power rate 283kW 40.6m 3 /h

14 The Effi Low Res ORC unit

15 Geothermal and solar energy data Geothermal data: they were obtained from the low enthalpy geothermal well, at Lesvos island, Greece, at Polichnitos area utilizing the heat of an existing geothermal field. From the existing drilling, the geothermal fluid is pumped with a temperature of approximately ~85 o C, with a productivity of 35m 3 /h at a depth of 150m. Solar data: An installation to measure yield of solar panels of flat and vacuum type was developed in the area, so as to receive performance measurements of solar panels. Μeteorological data for the area of Psachna Evias were taken from a combination of measurements from TEISTE meteorological station and a leading meteorological software.

16 Measuring points The temperatures at the entrance and exit on both sides of the evaporator, the condenser, and the scroll expanders were recorded. Pressure was measured at critical points of the working medium network. The digital readings were recorded every 30 seconds by using an advanced DAQ system. The refrigerant flow was measured with a Coriolis flow meter.

17 Typical results Indicative measurements and estimated efficiency during unit operation with solar panels under summer and winter conditions Points Enthalpy Temperature Pressure Enthalpy Temperature Pressure kj/kg oc bar kj/kg oc bar Season Summer Winter 3 432,825 76,2 23, ,632 76,6 23, , , , ,389 4s 415, , ,936 34,8 8, , , ,912 32,9 8, ,619 34,4 23, ,71 34,4 23,636 nth,th e 9,14% nth,the 9,42% nth 7,30% nth 7,63% nm 6,20% nm 6,50% nel 5,80% nel 6,10%

18 Typical results Indicative measurements and estimated efficiency during unit operation with geothermal energy unde summer and winter conditions Points Enthalpy Temperature Pressure Enthalpy Temperature Pressure kj/kg oc bar kj/kg oc bar Season Summer Winter 3 433,109 76,2 23, ,916 76,4 24, ,297 38,0 8, ,291 36,0 8,386 4s 415,895 35,0 8, ,936 33,0 8, ,819 33,0 8, ,915 31,0 8, ,619 34,4 23, ,716 31,4 24,154 nth,the 9,28% nth,the 9,55% nth 7,45% nth 7,77% nm 6,32% nm 6,50% nel 5,91% nel 6,10%

19 Conclusions/1 As for the combination with solar systems: during winter, although the unit is performing better, the exploitation rate is reduced due to the short period of sunshine; during summer its presence is significant, at a time when thermal solar systems are becoming less efficient due to the low heat demand. Then electricity generation is important, even with this low efficiency and exploitation. If it is combined with a cooling production unit to meet the needs of summer air conditioning, then the device becomes very attractive and completely renewable.

20 Conclusions/2 Regarding the exploitation of low enthalpy (temperature) geothermal installations, the above are valid with the positive indication that these installations also, make the unit very productive during winter compared to the existing heating devices. During summer, when these devices become inactive, their further use and utilization is clear and by coupling them, with a cooling network for summer air conditioning optimize their exploitation period. As a conclusion, it could also be pointed out that the use of this technology is important for rural areas, because by its implementation, energy needs of greenhouse crops can be covered in remote areas without the need for electricity transmission, especially in low enthalpy geothermal areas where efficiency is a direct function of the mass flow of geothermal fluids.

21 Future steps Optimize the operation of the ORC unit and study its application for low temperature energy sources utilization such as waste heat or Renewable Energy Sources (RES). This comprises: Analysis and design of energy production cycles using waste heat and Renewable Energy Sources; Expander design with computational fluid dynamics (CFD); Optimize the performance of the whole device using evolutionary algorithms; ORC improvements and test operation. Development of a unit with higher capacity that will exploit the geothermal field of Polichnitos area at Lesvos island. An autonomous tele-air conditioning network will be developed in the area with production/distribution of thermal and cooling energy. Production of electrical energy via ORC and a small photovoltaic installation.

22 Experimental Performance Evaluation of a Small Scale ORC Power Unit Working with Low Temperature Energy Sources for Power Cogeneration Maria K. Koukou, Speaker mkoukou@teiste.gr, m_koukou@otenet.gr