Testing time for microturbine CHP

A deregulated energy market will change the conditions of energy production, and the economic balance between on-site production and centralised production may be altered, resulting in changing patterns of demand. Lead times for on-site technology are shorter than centralised production. As a result, on-site technology can respond more quickly to changes in the competitive market, and greater risks can be taken.

In addition, small-scale energy production can be adapted to the on-site conditions and a customer's specific needs. For example, it would be possible to enable the customer to utilise their own resources and gain short-term economic advantages from market fluctuations.

As a result of this development, Vattenfall has looked into the concept of HSG (High Speed Generation), and has built a demonstration plant.

One prerequisite that Vattenfall determined as being a requirement for small-scale technology was the development of IT to enable operators to remotely control the operation of power plants. The rapid advances in IT have made such developments possible.

HSG technology

The HSG unit is a single-shaft machine, with compressor and turbine mounted on the same shaft at the rotor of the generator. A gearbox is not required, making the unit very compact. The combustion air flows through the generator via a centrifugal compressor. The air then flows through a diffuser, and thence to a recuperative heat exchanger in which the air is preheated by the exhaust gases. The fuel is mixed homogenously with the compressed and preheated air prior to entering the combustion zone. This ensures accurate control of the combustion temperature, and minimisation of NO_x emissions.

The combustion chamber consists of a pilot and a main combustion chamber.

The turbine is radial with uncooled vanes, supported by two oil-lubricated bearings each side of the permanent magnet.

The HSG is a synchronous AC machine in which the rotor speed is the same as the frequency of generated electrical voltage. At full load, the speed is 90 000 rpm.

Demonstration plant

A research project at the Royal Institute of Technology in Stockholm began in 1986 to evaluate the HSG technology. This project was financed by NUTEK (the Swedish National Board for Industrial and Technical Development), Vattenfall and ABB. Volvo Aero Engines became involved as an active partner later.

In 1993, the work on the HSG changed in nature from development to commercialisation, and HSG Development AB was founded with Vattenfall, ABB and Volvo as part owners of the company. This resulted in the construction of a demonstration plant at Pappersgruppen, a company based in Gothenburg in Sweden.

Leif Liinanki, project manager for the demonstration plant, said: "A research project of this type incorporates three identifiable stages. First of all, there is the period before start up. It is in this stage that most of the problems turn up and have to be solved. Consequently, it is a very instructive period.

"This is followed by the demonstration phase. Since the technique is untried, it has to be demonstrated. This creates attention and curiosity, as well as the embryo for a future market.

"The last stage involves long-term testing. We know practically from the tests all we need to know already, but it still has to be verified. Some of the questions that have to be answered are: how the plant works over a longer period of time, integration with the electricity supply network, emission levels, what the cost of operation and maintenance is, and whether there is a need for operational monitoring."

The Pappersgruppen plant first became operational in 1997, and will be evaluated over a five year period (1997 - 2002). To date, the plant has accumulated a total of 4000 hours of operation, and is expected to gather 25 000 - 30 000 hours of operation during the five year evaluation period.

The plant is designed for continuous operation with a total operation time of 5000 hours in each heating season (September to April). There is no demand for a heating plant in the summer.

The plant generates 38 kWe and 70 kWth, with a total efficiency of 80 per cent. The electrical efficiency is 30 per cent, but it is said that this can probably be increased to 40 per cent with continued development of gas turbine technology.

Fuel supply is natural gas from the North Sea. The gas pressure that is delivered to the customer is at around 4.5 bar, but the supplier does not guarantee that the gas pressure will always be this high. Unfortunately, the gas turbine requires a pressure that is close to this value, so there is a need for an on-site compressor to guarantee that the pressure will reach the requisite level. For this purpose, a reciprocating compressor is used, and a pressurised storage tank is placed between the compressor and the gas turbine so that high pressure is always available, and the compressor does not have to work continuously.

The operating performance of the plant during the first evaluation phase has been better than expected. First, the heat output of the plant was improved, which was mainly due to the fact that the incoming water supply was colder than had originally been planned.

Second, the emission levels given off by the plant were significantly lower than had been anticipated, with hydrocarbon emissions being lower than the detection limit of 4.5 mg/MJ fuel.

The pros and cons of microturbines

Microturbines have the following advantages:

However, Vattenfall has concluded that more experience of long-term operation is still required, and the demonstration plant will continue, and will look at the effects that variation in outdoor temperature and part-loading have on performance. Operation and maintenance costs will also be investigated.

One conclusion that has been drawn is that on-site heat and power generation is not yet commercially viable in Sweden. This is because Sweden has very low electricity prices, and in order to be commercially viable, new generation techniques have to have very low costs. However, on-site production is likely to be popular where the cost of electricity is higher, or supply is less reliable.

In the long-term future, customers will demand that heat and electricity be produced in an environmentally friendly way. In the short-term, the study concludes that it is likely that small-scale CHP units based on gas turbines will only enter the market for special purposes.
Tables

Table 1: Operating conditions at full load
Table 2: Measured emissions at full load