AXIAL TURBINE FOR LOW-GRADE HEAT SOURCES

VAPOR EXPANSION TURBINE IN THE 500 KW TO 5 MW RANGE DEVELOPED FOR ORGANIC RANKINE CYCLE (ORC) APPLICATIONS

Interest in low-grade heat recovery has grown in recent years. A number of approaches have been proposed to capture and convert low-grade heat energy, traditionally viewed as not commercially useful, into productive electric power. Among the proposed solutions is the use of ORC technology.

ORC is a vapor power cycle named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change taking place at a lower temperature than the traditional fluid of water-steam. This lower temperature phase change is what allows for the capture and conversion of low-grade heat from wasted thermal energy to usable electrical energy.

Aside from the use of an organic compound as the working fluid, ORC functionally resembles the steam cycle power plant: a pump increases the pressure of condensed liquid; the high-pressure liquid is pre-heated via a recuperator exchanger; the liquid is vaporized by extracting waste heat from the heat source through a heat exchanger; the high-pressure vapor expands in a turbine that drives a generator thus producing power; and the low-pressure vapor leaving the turbine is de-superheated through the recuperator and condensed before being sent back to the pump to restart the cycle.

A key component of the cycle is the vapor expansion turbine. The maturity of this turbomachinery, in large measure, defines the commercial viability of an ORC solution. Today, small-scale positive displacement units in the tens or few hundreds of kW capacity are available to function as the ORC driver.

Additionally, oil and gas providers can, with little modification, supply commercially viable turboexpanders suitable for ORC applications at the larger sizes (>5-7MW). However, many of the available heat sources do not line up with this supply, and as a result, ORC applications that are in the middle scale of the above ranges have struggled to find their footing in the market place.

To satisfy this market need and supply ORC solutions in the 500kW-to-5MW range, an axial turbine has been developed that is capable of supporting applications with low-grade heat sources found in geothermal projects, as well as moderately higher temperatures found in industrial waste heat segments (Figure 1).

ORC’s appeal in tackling the low-grade heat problem is largely due to its broad applicability. With minor modifications, the technology applies to geothermal, biomass, solar-thermal, reciprocating and gas turbine exhaust and industrial process heat. The design process followed the Design for Manufacturability and Assembly (DFM/DFA) method, which aims to yield product designs with reduced part count, simple manufacturing techniques, and standardized parts and materials.

Process conditions required that the turbine be suitable for applications at the previously stated power ranges, using TAS Energy’s primary ORC working fluids (R245fa and R134a), at typical applicationdefined temperatures (200°F – 500°F). These conditions create a large range of flows, enthalpy drops, pressure ratios, and optimal design speeds, which are problematic for a standard design, compliant with DFM/DFA principles.

The range of flows is covered with two cast turbine plenums — one for low flows and one for high flows. The plenum castings include three optional inlets regardless of actual project design requirements. Thus, with two plenum designs, six increments are available for optimal flow distribution into the plenum by machining out the required number of inlets. The plenum’s stiff design reduces the need for expansion joints.

The varying temperature range and fluid selection tend to trend together and this range of requirements is solved by having a singlestage or two-stage turbine that fits within the plenum castings. The single stage is applied to the lower-to-moderate temperature ranges, and the two-stage units cover the higher temperatures. To reduce lead times, the turbine is configured for wheel size and speed to accommodate off-the-shelf gearbox units and parts including rolling element bearings. The bearing design eliminates the need for thrust bearings in the gearbox, which improves gearbox efficiency to greater than 98%.

When needed, the second stage is mounted on a shaft extension designed into the common subassembly. The subassembly is fixed for all units and houses a standard set of bearings, seals and shaft. The blades are electrochemically milled (ECM) on the rotor disk, and the shroud is electron beam welded. The turbine rotors are slip-fit and locked to the shaft with tapered locking devices to allow rapid removal and replacement.

Geothermal deployment

Terra-Gen Power, LLC’s geothermal flash plant at Beowawe, near Battle Mountain, NV, uses this technology — a result of the U.S. Department of Energy’s (DOE) Geothermal Technologies Program (GTP). The goal was to prove the technical and economic feasibility of electricity generation from nonconventional geothermal resources using a 199°F heat source, and become the first commercial use of a low-temperature bottoming cycle at a geothermal power plant.

This ORC plant generates 2.5 MW gross and has been running for more than two years. It augments the original flash steam power plant by 10% without consuming additional geothermal resources (Figure 2).

The axial turbine was also incorporated into the design of an ORC project in Western Turkey. However, this site, comprised of two plants, is a high-temperature, mixed-enthalpy resource (steam and hot water), typical of most geothermal systems in Turkey. Each plant is 7.0 MWe gross capacity, which is achieved by providing two turbines, each driving clockwise and counterclockwise through their respective gearboxes to both ends of a common generator. The first plant is due to be commissioned in the spring of 2013 with the second soon afterwards.

Author:

Ryan Elliott is Manager of Renewable Energy Systems Applications at TAS Energy. He is responsible for driving the technology choices that go into ORC systems. He has a BSME from Texas A&M. He can be contacted at 713 440 4268 or relliott@tas.com