|The first production unit of the EPS100 7.5 MWe heat engine is completing factory checkout tests at Dresser-Rand|
Echogen Power Systems (EPS) has developed a supercritical CO2 (scCO2) Rankine Cycle that uses carbon dioxide in place of water/steam for efficient cogeneration or an improved conversion of waste heat into electricity, reports Alex Kacludis.
Echogen’s Thermafficient Heat Engine uses scCO2 and patent-pending operating cycles to deliver a flexible, low-cost thermal engine for a wide variety of applications. The engine has five main components: exhaust and recuperator heat exchangers, condenser, system pump and power turbine.
Ancillary components (valves and sensors) provide system monitoring and control. Heat energy is introduced to the power cycle through an exhaust heat exchanger installed into the exhaust stack from a gas turbine or reciprocating engine, or into a flue gas stream from a fuel-fired industrial process.
Echogen’s technology recycles the wasted thermal energy and provides integrated power and heating or cooling with flexible system architectures, configurable for power, cogeneration or trigeneration. Supercritical CO2 is a low-cost fluid that is non-toxic and non-flammable. The high fluid density of scCO2 enables extremely compact turbo-machinery designs and permits the use of compact heat exchanger technology.
The high thermal stability and non-flammability of CO2 means the exhaust heat exchanger can be in direct contact with high-temperature heat sources, eliminating the cost and complexity of an intermediate heat transfer loop typically used in Organic Rankine Cycle (ORC) applications. The Echogen Cycle enables single-phase heat transfer, which makes the heat exchanger more effective, and cuts the size and cost of the exhaust heat exchanger.
Echogen is currently building the EPS100, a 7.5 MWe thermal engine for large industrial, fuel-fired processes, utility-scale power generation and concentrated solar thermal utility applications.
The EPS100 uses an scCO2 turbine generator and incorporates an advanced power cycle to maximize exhaust thermal energy utilization by reducing the exhaust temperature to a minimum practical limit. Because the EPS100 power turbine is a separate unit, two different options for the turbine are offered, one a high-speed, single-stage radial turbine, the other an API-compliant lower speed axial turbine.
A second system platform, the EPS5, is a 300 kWe thermal engine based on Echogen’s 250 kWe demonstration system tested at the American Electric Power (AEP) Dolan Technology Center during 2010–11. The EPS5 utilizes a turbo-alternator and is designed for industrial and distributed generation applications.
scCO2 with gas turbines
Across the United States, utility companies are turning to natural gas to generate electricity, with 258 plants expected to be built between 2011 and 2015, according to the US Energy Information Administration. The US will add 222 GW of generating capacity by 2035, equivalent to 20% of current US capacity, or 58% of all of the expected new power generation to be added, according to the agency.
|The EPS5 300 kWe heat engine, which is derived from the 250 kWe demonstration system (above), has completed checkout testing and is now in endurance testing|
Historically, natural gas-fired combustion turbines have been used by utilities to provide both baseload and peaking power generation. Typically, systems rated over 100 MW are used in baseload operations while smaller gas turbines handle peaking and mid-merit capacity. But changes in the power industry along with new Environmental Protection Agency rules and technological advances mean gas turbines are used increasingly for baseload power in combined-cycle systems. For instance, gas turbines accounted for only 15% of power generation in 1998 but are expected to provide 40% of US power capacity by 2020.
A 2009 Forecast International study estimates the global installed base for industrial gas turbines at 46,455 units, of which 33% (15,330 units) are heavy-frame gas turbines, 21% (9755 units) are aero-derivative, and 46% (21,370 units) are light-frame units.
Particularly on larger units, the gas turbine is often combined with heat recovery steam generators (HRSGs) to recycle usable (waste) heat in the turbine exhaust streams for cogeneration or bottom cycling to increase system efficiencies from the typical 35–40% for simple-cycle turbines to over 60% for combined-cycle systems. But unfavourable economics have prevented smaller systems from deploying a combined-cycle architecture.
In 2011, Echogen conducted a study that compared the Echogen EPS100 heat engine’s performance with that of a comparably sized, double-pressure HRSG (DP-HRSG). The study found the EPS100 system’s power output versus ambient temperature outstripped that of single-pressure steam systems and is comparable to that of a double-pressure steam system.
The Echogen system can increase net power production from heat in gas turbine exhaust. For example, the net power on 20–50 MWe gas turbines can be increased by up to 35%, comparable to a DP-HRSG but at a lower cost for installation. All study cases for the EPS100 heat engine assume an evaporative cooled system condenser. For most climates, the baseline cycle provides a good balance of performance. For high ambient temperature climates, especially where water restrictions are an operating constraint, a high ambient, fully air-cooled version is under development.
sCO2 with reciprocating engine gensets
The traditional approach of building large centralized power plants to address the increasing demand for electrical power is often hindered by social, economic and environmental constraints. Distributed generation (DG) has emerged as a desirable option for adding capacity through relatively small generating units (typically less than 30 MWe) at or near consumer sites.
Distributed generation units can provide incremental capacity at relativity low capital cost and can be brought on line faster than centralized power systems. For DG applications, reciprocating internal combustion engines fuelled by natural gas or diesel fuel have become widespread (see Table 1).
Typical DG applications include: natural gas compressor stations, on-site gensets at industrial facilities, standby or emergency back-up units for large institutional facilities, and small (<25 MW) gas turbine-based and multiple reciprocating engine-based electricity generation plants for remote and rural locations.
While DG offers advantages, the relatively small size of equipment means overall efficiency falls below that of larger centralized power generation. Much of the fuel energy is unused and escapes as waste heat.
Heat can be captured and used to provide thermal energy to the local site, but local demand for heat is often much lower than for electricity. Electrical power usually remains the most in-demand product of the DG system. Converting relatively low-grade thermal energy to electrical power is traditionally accomplished through heat recovery steam systems.
While extremely successful at utility scales, the cost and performance of steam systems generally becomes unfavourable at the scales common in DG. But the scCO2 cycle scales well into smaller sizes from both a performance and economic perspective for bottom cycling reciprocating engine gensets.
Supporting remote locations
In northern Canada, a remote community relies on a 6.5 MWe electrical power collective, containing four 1.05 MWe and two 1.13 MWe reciprocating engine gensets fuelled by natural gas.
Three gensets typically operate to provide 3.2 MWe baseload while maintenance is performed on the second set of three units. For six months, coinciding with spring and summer, all units operate to provide up to 6.5 MWe of peak power to support the additional demands of the community’s local fishing and canning industry.
Results of a waste heat to power analysis using an sCO2 heat engine for bottom cycling for each type of reciprocating genset are summarized in Table 2.
In an increasingly competitive environment, manufacturers are seeking to cut their costs. Fluctuating energy prices often channel this investment into cost-effective energy-saving technologies and practices that will reduce operating costs while maintaining or increasing product quality and yield.
Energy-efficient technologies often bring other benefits, such as higher productivity or environmental gains, reducing the regulatory ‘burden’. Waste heat can be captured from many industrial processes through waste heat recovery technology. For large energy consumers in industry, waste heat recovery opportunities are found in steam generating and direct-fired heating processes such as furnaces and kilns. Prospective industrial customers include chemical processing, oil and gas exploration and transmission, petroleum refining, iron, steel, glass, cement, pulp and paper, and power generation, typically operating with large sources of energy loss from hot exhaust gases and residual heat in liquid product streams.
Waste heat recovery represents the greatest opportunity for reducing energy loss in these industries while simultaneously reducing their carbon footprint and associated greenhouse emissions with improved overall energy production efficiency. An scCO2 heat engine with a waste heat exchanger installed into the hot process exhaust duct can enable industrial users to repurpose this emission-free energy to the facility’s internal power grid to drive large process fans, blowers, pumps or motors, or sell it to the grid to support clean energy production, distribution and use to enable their local utility to meet their Renewable Portfolio Standards.
The outlook for scCO2
Supercritical CO2 heat engines are scalable across a broad system size range, from 250 kWe to 45 MWe and above, with net electrical output to support the widest possible variety of industrial and utility-scale applications.
The sCO2 Cycle is thermal source neutral − suitable with a wide range of heat sources from 200°C to 500°C with efficiencies up to 30%. New energy production can be offset with recovered energy without increasing greenhouse emissions while improving overall energy production efficiency. The scCO2 heat engine can add up to 35% more power to simple-cycle gas turbines, 10–15% more power to reciprocating engines, and can significantly improve the energy efficiency and bottom line performance at steel mills, cement kilns, glass furnaces and other fuel-fired industrial processes by converting previously wasted exhaust and flue gas energy into usable electricity.
Alex Kacludis is an Application Engineer at EPS LLC; www.echogen.com