MiniGen™ gets into a growth business

Efficiency improvements derived from producing energy at the point of use in distributed, combined heat and power (CHP) systems have long been lauded by governments, environmentalists and industry alike. However, a major impediment to the widespread introduction of CHP has been expensive emissions control equipment, which until recently made it only really economically viable for comparatively large installations. Now though, microturbines may open the possibilities of cogeneration to a whole range of smaller applications. And, unlike some other alternatives to the reciprocating engines in common use today, this is a fully commercial, working system.

Technology alliance

In late 1999, BG Technology formalised a long-standing alliance with Capstone Turbine Corporation and began marketing a CHP system, based on the Capstone microturbine, under the name MiniGen™. They offer modular systems from 30 kWe up to 500 kWe and a 60 kWe unit is under evaluation prior to its launch, due in the UK next April. The company had also intended to develop units in the 1 kWe range, based on the Stirling engine through a similar technology agreement, although the future of the domestic systems is uncertain and a launch date has yet to be announced. In the UK, the first fruit of the Capstone / BG Technology union has been installed at the WJ Findon horticultural site near Stratford upon Avon, where the unit supplies heat, lighting and CO2 for improved plant growth. The result is that almost the entire gross combustion product is used.

Power characteristics

The maximum output of the system is to some extent dependent on the prevailing atmospheric conditions, but is limited to 30 kWe or 96 000 rpm, whichever comes first. It also delivers some 60 kWth. The unit may be used for base-load, but is also well suited to backup or emergency applications. From a cold start, it can be up to full power within a few minutes and can be kept spinning through the minimal use of the three fuel injectors, requiring just a few moments to accelerate the turbine up to full speed. A fairly flat efficiency curve is also claimed - generating as little as 7 kWe before it drops to anything below agreeable levels.

Central to the success of the system is its solid-state control unit and associated electronics. Capstone developed the proprietary system side-by-side with the combustion unit and fully one third of its staff work on the control aspects of the unit. The system may be accessed via modem and anywhere that a GSM telephone can receive and transmit data from a computer can be the control room, where a simple Windows-based display is the interface. Where multiple units are used, the control system allows a single unit to operate at lower demand levels before ramping up with other units as demand rises. The units mimic phasing, either with the wider transmission grid or in an island situation with other MiniGens™.

Emissions

The use of air bearings to support the turbine, eliminates the need for conventional oil or water lubrication and thus dramatically improves the emissions levels. The lean burn turbine at the heart of the system combined with a comparatively low combustion temperature produces significantly lower emissions of NOx than conventional installations such as diesel gensets. Figures show the microturbine emitting <9 ppm of NOx using natural gas. Furthermore, MiniGen™ produces low levels of carbon monoxide, allowing it to be used directly as a feed stock for the carbon fixing occupants of the greenhouse.

The multiple fuel capability, in the case of many gaseous fuels allowing the units to change without any adjustments at all, opens a number of other applications. For instance, where mobile units are required, quick- release jacks may be used to enable rapid installation. The system is also capable of utilising relatively poor quality fuels, such as those from landfill sites. Even when changing from gas to liquid fuels, such as diesel or kerosene, only limited adjustments to the three injectors are required, along with a change to the fuel compressors and pumps.

Reliability and maintenance

The Findon system was installed in January of this year and, to date, no operational difficulties have been experienced. Regular servicing consists of replacing the combustion air filter at 8000 hours of operation and washing the filter for the control unit. During the second year the thermocouple sensing devices are expected to replaced as a precautionary measure, or typically after 16 000 hours of operation. The unit is expected to have a lifespan of over 40 000 hours or 10 - 15 years. BG Technology offers a comprehensive maintenance programme with the installation that is costed at something less than 1p per kWh of generation. At around a fifth of the costs for reciprocating engines, this maintenance figure compares favourably. BG Technology also offers a warranty agreement to replace the unit, and is confident in its guarantee of a minimum 90 per cent availability.

Rates and returns

Although a comparatively high initial capital expenditure is required – £800 to £1200 per installed kW of capacity – the manufacturers claim a typical return on the investment of three to five years. Fuel costs per kWh of generation are roughly three times gas costs. Typical UK prices would give 2.5p/kWh. However, this does not include the additional potential benefits from the high grade and low grade heat which the system can produce.

Despite its advantages, CHP does not hold all the answers, for example at Findon’s an over-supply of heat may make it impossible to utilise CHP for all the electrical energy requirements. Nonetheless the MiniGen™ system extends the range of applications that may take advantage of the benefits of cogeneration. David Findon, runs a small independent business and, concurs. Having already invested considerable capital in the device he said of the installation, "I am beyond impressed". Praise indeed.