Distributed Energy Systems
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CEC has been working on Brayton cycle power generators for use in commercial and residential Combined Heat and Power (CHP) systems. This is an area with extremely large market potential based on economic necessity, since this type of system can save commercial users virtually their entire electricity bill. As energy costs are driven up by increasing demand, the argument in favor of distributed energy generating (DEG) systems will become increasingly compelling. A new factor that is growing in importance centers on concern for global warming which is thought by most to be exacerbated by the release of greenhouse gasses into earth's atmosphere. Ultimately the waste of two out of every three calories burnt to deliver electricity to a customer will no longer be sustainable. At that point the demand for DEG will number in the hundreds of millions of dollars annually. Coal-fired electricity generation in the United States emits on average about 2.1 lb/kWh of CO2. Oil-fired generation contributes an average of 1.97 lb/kWh of CO2. Natural Gas-fired generation contributes 1.32 lb/kWh of CO2, the least carbon-intensive fossil fuel available.
Apart from the Pacific Contiguous Division where the CO2 emission rate is below 0.5 lb/kWh, the division with emission rates above 1.0 lb/kWh CO2 predominate. The makeup of the CO2 emissions in each Division as tabulated on the page 5 shows avid use of coal and oil burning power plants in these areas. Were any U.S. industrial or large commercial company to install gas-fired combined heat and power (“CHP”) plants at each of its facilities, it would reduce its aggregate CO2 contributions from around 1.2 to less than 0.44 lb/kWh; a reduction of 63%! – a figure well beyond the 7% reduction now targeted by Kyoto. An important part of the economic case for the optimum economic use of gas turbines in a multi-function energy systems which are subject to wide load swings in non-coincident heat and power demands, is that the power plant has to be extremely tolerant of part load operations. This militates strongly in favor of closed loop the Brayton cycle since it offers high, flat, efficient part-load capabilities. |
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CEC's system utilizes almost identical first and second stage turbo machines to the units used in the air pressure system for the Ballard 250 kilowatt stationary fuel cell. These turbo machines have been tried and proved to be durable and reliable in extensive field trials extending over several years in numerous locations in Europe, Canada and the US. Ballard's goal with these field trials was to operate each of 15 complete systems without shutdown for one year. The first of these systems has reached its goal and has been taken out of service for evaluation. The turbochargers, after approximately 10,000 hours of operation, have been returned for teardown inspection. Both are in excellent condition and require no more than cleaning to be returned to service. [endurance] The turbo alternator is driven by a turbine identical to the low-pressure stage and consists of a high-speed, wild-frequency alternator. The stator closely resembles those used in similar size micro turbines, including the 60kW Capstone unit. Commercially available solid state power rectification and inversion systems are used to regulate power output. The combination of a closed loop and the high rotational speeds of the two stages results in extremely quite and low vibration levels in operation. The modest intercooler design requirements are well within existing commercial practice for cross-flow, plate-fin heat exchangers since the low pressure compressor is operating at a modest pressure ratio of 2.5 to 1. T he recuperator is a fairly standard counter-flow, steel, plate fin unit.External combustion is provided by a cylindrical surface burner charged with a mixture of air and natural gas at standard line pressure. A combined radiant and convective design is used with 80% excess air mixture resulting in a flame temperature that produces less than 2 parts per million of NOx to ensure ultra-low emission characteristics for the system. Heat from the combustor is exchanged with the working fluid in a simple cylindrical design with fin surfaces on both sides of the cylinder. Combustion air leaves the system through a small recuperator to heat the incoming combustion air.The hot air from the heat sink exchanger is used for space and water heating and may be used to drive absorption chillers for air conditioning. A proprietary control system is used which is self-modulating and remarkably simple. The conversion efficiency of the power unit is nearly flat at about 40% over the output range of 12 to 60 kilowatts. Below 12 kilowatts power is available at slightly lower efficiencies. The three concerns for transient response are changes in thermal demand from heating and cooling systems, sudden, large changes in power demand, and induction motor start-up loads.T hermal demands are the simplest to handle, because the combustor heat exchanger is kept at essentially constant temperature and its thermal mass acts as a thermal capacitor for sudden thermal load changes. Control of the heat to the air-conditioning and heating system are handled by simple air valves to adjust the flow and temperature delivered. If the required heat is not available because of operation at low power demand, the turbine inlet temperature is lowered by bypassing some of the air around the combustor (kept at constant temperature) which will result in the pressure ratio control raising the system pressure and thus providing the required heat.The electrical power control is totally independent from the thermal demands. Sudden large changes in power demands are the most difficult to handle. Since the system operates at essentially constant shaft RPM and constant combustion temperature, it requires a very fast fill and drain system to adjust the system operating pressure. The unit will handle an electrical load swing from minimum to maximum power output in half a second. This lag is enough to "dim the lights" and a small amount of electrical storage capacity is provided to maintain constant output voltage. |
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System Sizing:
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Distributed CHP power systems apply to a market in which one third of the power is consumed by a small number of large industrial customers, another third by a much larger number of commercial enterprises which range in size from large to small, and the last third by a vast number of residential users. CEC considers small-scale CHP systems to be those with electrical capacities below 300 kilowatts. Current CHP systems cater mostly to larger applications. The challenge is to extend the advantages of cogeneration to the preponderance of small-scale energy users. These include hospitals, shopping malls, restaurants, food markets and the most populous of all, residences. Distributed power systems for these applications require major advances in small, cheap, modular, quiet, safe, efficient, and low-emission energy production technologies. CEC seized upon this opportunity to address the market segment comprised of small-to-medium commercial businesses; particularly those with long operating hours and intensive energy use since they are most sensitive to high utility rates. The largest market segment remains residential use, particularly in rapidly developing economies that still lack adequate electric power infrastructures, where expanding energy demand in the face of growing environmental concern drives them toward greater use of natural gas. At the residential level of use in these environments the right distributed energy solution could create a watershed away from expensive central power economies toward economies based on ubiquitous small, efficient, low-emission, distributed energy systems. |
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Potential Market Size:
Most CHP systems marketing professionals agree that an inexpensive gas turbine with a thermal efficiency greater that 40% over a load range of 15 to 300 kilowatts offering ultra-low emission characteristics configured ideally for CHP applications would be in great demand. |
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US Domestic Market Entry:
According to US Government figures as analyzed by Regional Economic Research, Inc., a peak electrical load of 8 watts/sq.ft. in food stores in the U.S. Southwest occurs at about 3 PM on summer days. In the 120,000 sq. ft. store in the example this equates to 960 kilowatts. By employing 80 kilowatt systems, the peak requirement would call for 12 units, which provides ample redundancy margin to operate normally without external power backup. Based upon California electricity and gas prices, a leased group of such systems would produce a net annual saving of over $800,000 for the store's owner.
The same rationale can be successfully be
applied to hospitality establishments, medical clinics, department stores,
and many other commercial service facilities.
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Technical Approach
CEC's design specifications calls for a turbo generator that will:
use natural gas at a standard distribution line pressure of 5 inches of water;
be priced at below $400/kW to enable economical deployment in clusters to ensure uninterruptible power;
offer sufficient capacity to bear all heating and cooling loads with rejected heat;
ensure NOx emissions below 2 parts per million;
operate with
require
operate
have a
For a more fundamental review of CEC's distributed energy systems rationale, select MORE
In
1999, the national average thermal efficiency of fossil fuel power plants
was estimated by the DOE to be 32.54%. From this must be subtracted
the energy losses of transmitting and transforming the power which averages
about 8%. Hence, for every unit of electrical energy used by a J & J
facility, more than three units of fuel energy are consumed. The map shows
the average CO2 emission rates in each of the nine Census
Divisions in the country.
The intent is to enter the CHP market by
focusing on specific commercial segments in which economic circumstances are
highly favorable for migration to self-owned distributed energy systems. One
such application would be in food retail stores and supermarkets.
