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2 GRC Transactions, Vol. 30, 2006 An ORC Power Plant Operating on a Low-Temperature (165 F) Geothermal Source Dr. Frederick J. Cogswell United Technologies Corporation, East Hartford, CT cogswefj@utrc.utc.com Keywords Organic Rankine Cycle, ORC, geothermal power, UTC, UTC Power, r134a, r245fa, low-temperature, oil/gas, Chena Hot Springs ABSTRACT Organic Rankine Cycle power production from low temperature resources has inherently a low thermal efficiency. Low efficiency requires increased power plant equipment size (turbine, condenser, pump and boiler) that can become cost prohibitive. The use of ORC power plant hardware derived from air-conditioning equipment overcomes this cost problem since air-conditioning hardware has a cost structure almost an order of magnitude smaller than that of traditional power generating equipment. Using the HVAC derivative concept, a low-cost 230 kw ORC power plant has been developed. This paper describes the design and qualification testing of an ORC unit at UTRC that is suitable for low temperature, geothermal energy sources. Introduction Observations of centrifugal chiller dynamics at Carrier Corporation led to the idea that standard chiller hardware may be run in reverse to generate power. The Carrier 19XR chiller line uses pipe diffusers at the exit of the compressor to recover pressure and thus achieve better efficiency. When run in reverse the diffusers act as turbine nozzles that accelerate the refrigerant to supersonic velocities. The development of this system is further described in Reference 1. In 2004 United Technologies Corporation, through their UTC Power Division, released a 200 kw net power generation ORC system. Figure 1. Drawing and installation of UTC air-air ORC unit. 729 This unit, shown in Figure 1, has an air-cooled condenser and air-heated direct-expansion evaporator (Reference 2). A revised ORC system was developed at UTRC that is targeted for the geothermal and oil/gas markets where the heat resource exits either as hot water or low-pressure steam. This system has a water-cooled condenser and water-heated poolboiling evaporator. A prototype unit was developed for a geothermal application at Chena Hot Springs, Alaska. The design, qualification testing, and site operation of this unit is described in this paper. Also described in this paper are applications of this unit to other geothermal or hot water sources. The design goal of the United Technologies ORC systems is to minimize product cost in terms of $/kw-hr of net power generation. The first cost is minimized by the following: o Use of Carrier standard parts wherever possible utilizing Carrier s mass-production cost structure. The major components (turbine/generator, condenser, and integrated evaporator) are all in mass production in Carrier factories o Minimization of modifications to the Carrier standard equipment as much as possible.. The installed cost is minimized by: o Delivery of a single skid with minimal required site work. o A standard chiller factory design package requiring field connections only to the heat exchanger water-boxes and a single power connection.

3 The current embodiment of the UTC ORC system produces a net power of 200 kw for air-cooled and 230 kw for watercooled systems. This capacity, along with the easy skid-mount installation, allows for distributing the ORC unit at well heads. For many geothermal sites the wells are dispersed, and it is easier to run electrical wires to each well than it is to run water lines from each well to a centralized power-generation unit. Liquid-Liquid ORC Hardware Description The liquid-liquid ORC unit is shown in the UTRC CHP test facility in Figure 2. The major components are labeled. They are: Component Manufacture Comments Turbine/ generator Charlotte plant Modified 19XR2 chiller motor/compressor. Two pass. Condenser Float valve assembly removed. Contains sub-cooling tube bundle that Charlotte plant helps the refrigerant pump avoid cavitation. Evaporator Refrigerant pump Pump inverter Motor starter and power electronics Control electronics Houston plant Roth ABB, Carrier supplier Benshaw, Carrier supplier Controller by Carrier Contains integrated pre-heater bundle on second pass of the water. Solid state starter that is used on chillers. Also contains power distribution to other components and a transformer. Programmable controller set with 24 inputs and 24 available outputs. In an ORC system, as liquid refrigerant is pumped from the low pressure at the outlet of the condenser to the high pressure of the evaporator, it becomes highly sub-cooled. The sub-cooled refrigerant must be heated to saturation before it can boil. The heat required for pre-heating can be almost as large as the heat required for boiling (see Figure 5 below for the PH diagram for the Chena application). Some ORC systems use a separate pre-heater to accomplish this task. The Carrier evaporator includes a pre-heater tube bundle in the same evaporator shell, eliminating a separate vessel and achieving a simpler system design and assembly. Since the refrigerant entering the suction side of the refrigerant pump is near saturation, it is subject to pump cavitation during system transients. Some ORC systems install the refrigerant pump in a hole, or raise the condenser to a great height to achieve more liquid head at the pump entrance. The Carrier condenser includes a sub-cooling bundle in the bottom of the heat exchanger shell which adds approximately 5 F of extra sub-cooling. This alleviates the need for a large height difference between the condenser outlet and the pump inlet. ORC System Efficiency The efficiency used to rate an ORC system depends on the application. There are two standard efficiencies used: o Thermal Efficiency is defined as the net power (P net ) out divided by the heat into the evaporator(q hot ): P Q net hot Figure 2. Picture of Geothermal Prototype installed in the UTRC CHP test facility, with major components labeled. o Utilization is defined as the net power out divided by the amount of heat that would be available if the heat source were cooled to ambient. The Carnot thermal efficiency of a machine operating between two infinite reservoirs, a hot one at T h and a cold one at T c, is simply 1 Tc Th. The efficiency of an ideal/reversible machine operating between a limited stream of hot fluid (with constant specific heat) and an infinite cold reservoir can be expressed as a function of the entering and leaving temperature of the hot stream, and the cold reservoir temperature. The change in entropy of hot stream: Sh = m h cp ln ( Th _ in Th _ out ), is equal to the entropy flow into the cold reservoir: Sc = Q c Tc. The thermal efficiency can Tc ln ( Th _ in Th _ out ) be solved to be: = 1 Q h Th _ in Th _ out Th _ in Th _ out Tc ln( Th _ in Th _ out ) and the utilization to =. T T T ln T T max h _ in c c ( h _ in c ) Figure 3 shows the thermal efficiency and utilization for a reversible machine operating with a hot stream at 165 F and cold reservoir at 40 F (the Chena condition) for various leaving hot-water temperatures. The thermal efficiency is maximized (at the Carnot reservoir value) when the utilization is zero. At full utilization the maximum efficiency is approximately halved. For a geothermal site the available hot water flow rate and ground-source temperature are generally prescribed. Of importance to the customer is utilization, not thermal efficiency. Geothermal ORCs are designed to have high utilization at the expense of thermal efficiency. For a given ORC power generation system, the utilization is increased by decreasing the hot water flow rate. The evaporator must be designed to work well at lower flow rates. When an ORC system is used as a bottoming cycle for other power generation systems (such as a reciprocating engine), the 730

4 that the vapor leaving the turbine and entering the condenser is sufficiently dense to avoid excessive pressure drops and/or choking. Typically the low-side pressure is between 30 and 80 psia. Reducing the low-side pressure below this range would require larger, more expensive hardware to handle the flow. Figure 3. Ideal efficiency and utilization for a device operating with a limited hot resource. (Thot_int = 165 F, T_cold = 40 F) Also shown is an ORC test point at 45% of Ideal. design philosophy may change. One possible application for this same product is waste-heat recovery from reciprocating generator jacket integration. In this type of closed system the heat-available is prescribed and the hot water flow rate may be varied to maximize the overall system efficiency. A higher flow rate is generally used to keep the thermal efficiency of the ORC system higher. Chena Unit Design The geothermal source at Chena Hot Springs is characterized by 1060 gpm at 165 F. A cold source is also available at 1500 gpm at 40 F year-round. Since the temperature difference is over 100 F this is a viable site for geothermal power generation. The refrigerant r134a is ideal for this application. Figure 5 shows the design conditions for the refrigerant and water. The saturation temperature in the condenser is 52 F. The saturation temperature in the evaporator is 145 F. At these temperatures r134a produces a high and low-side pressure of 260 and 62 psia respectively. These pressures meet the criteria mentioned above. Hot water: In = 165 F; Out = 147 F; Flow = 1000 gpm Cold water: In = 40 F; Out = 50 F; Flow = 1700 gpm ORC Working Fluids An ORC working fluid is chosen to produce proper pressures in both the high-side (evaporator) and lowside (condenser) of the system. Two refrigerants have been tested in UTC ORC systems: r134a and r245fa. Figure 4 shows the saturation curves of these two refrigerants. Since it is desirable to use commercially available turbine and evaporator hardware from Carrier factories, a maximum working pressure design limit is imposed. Currently this limit is at approximately 300 psia. Increasing this limit would require greater hardware expense. If r134a is chosen as the working fluid the maximum boiling temperature is ~155 F, and if r245fa is chosen then the maximum boiling temperature is ~255 F. The low-side pressure must be low enough to produce a turbine pressure ratio of at least three, but is high enough so Figure 5. Design point for Chena Hot Springs; refrigerant PH and TS diagrams. CHP Facility at UTRC The UTRC CHP facility permits the evaluation of new products and technologies. This facility was modified to accommodate the ORC geothermal unit. Hot water is provided by a run-around loop which has a steam-injection valve. 120 psig steam is injected to raise the temperature of the water before it enters the evaporator. An expansion tank with a steam-trap valve located downstream of the evaporator allows the excess condensation water to drain from the system. The steam system has the capacity to inject 1000 tons of heat (12,000,000 btu/hr), although only 700 tons were required for the Chena application. The facility uses a standard cooling tower loop with bypass to cool the condenser water. Figure 4. Saturation properties for r134a and r245fa, with design limits. Qualification Test Results The first liquid-to-liquid ORC unit, shown in Figure 2, was build by UTRC and tested in the UTRC CHP facility. This unit was run for 1000 hrs and through a compete set of qualification tests. The qualification tests included performance and boundary testing, emergency stops, loss of cooling water, loss of heating water, and other system faults. No instabilities were found. The system operated stably down to 25% of full power. After

5 hrs of operation the unit was torn down, inspected, and then reassembled for its final tests. The last 50 hours consisted of 42 grid-outages (where the 600 amp disconnect switch feeding the unit was suddenly opened during operation), and debris/erosion tests (where stainless steel debris ranging in size from 73 to 1500 microns was placed in piles around the turbine before operation, and <70 micron debris that can pass through the filters was injected at the turbine inlet during operation). Finally the unit was torn down and re-inspected. No significant damage was found. Figure 6 shows the turbine bearings disassembled for inspection, and the turbine wheel after the debris tests. Figure 8. System pressures during startup, operation, and shutdown. Figure 6. Turbine components inspected after endurance and qualification tests. No marks on the bearings, no FOD damage to the turbine wheel. The off-design performance is shown in Figure 7. The plot on the left shows the change in generation capacity as a function of varying the high-side pressure. This variation was achieved at the UTRC CHP test facility by changing both the entering hot water temperature and the hot-water flow rate. In an actual geothermal site the high-side pressure can be changed by varying the hot-water flow rate only (since the well temperature will vary only slightly over very long periods of time). Figure 8 shows system pressures during startup, operation, and shutdown. A variable speed hot water pump is used at Chena both to startup smoothly, and to vary the system power production. shown in Figure 7, combined with model predictions, provides a high degree of confidence that 230 kw will be produced with the 40 F entering condenser water temperature available at Chena. Installation and Operation at Chena Hot Springs The unit is scheduled to be shipped to Chena in June of 2006, and commissioned during July. A second unit will be subsequently built and delivered to Chena for a total net capacity exceeding 400 kw. Acknowledgement We acknowledge the DOE Geothermal Resource Office for their generous support of this project through contract # References 1. Brasz, J.J., Holdmann, Gwen, Power Production from a Moderate -Temperature Geothermal Resource, paper presented at the GRC annual meeting, Reno, Nevada, September 25-28, products/purecycle/purecycle.shtm Figure 7. Capacity with variations in low-side(left) and high-side (right) pressures. Figure 7 also shows the variation in generated power with changes in low-side pressure. The low-side pressure was changed by varying the condenser water flow rate. The lowest water temperature that the facility cooling tower could achieve at full power was 48 F. However, the very linear data trend Abbreviations CHP ORC UTC UTRC Combined Heat and Power Organic Rankine Cycle United Technologies Corporation United Technologies Research Center 732