Figure 1. Eighteen Jenbacher gas engines are being installed on the Isle of Wight to supply greenhouses with energy and carbon dioxide
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Austrian Engine manufacturer Jenbacher recently secured a contract to supply its gas engines to what is the largest ever combined heat and power project in the European horticultural industry on the Isle of Wight. PEi takes a look at this project and the rising popularity of the gas engine in the on-site power sector.

Too much choice can be a bad thing: weighing up the relative merits of diesel engines, gas engines and small gas turbines, for example, is a difficult task for any potential industrial autogenerator. A liberalized market that offers many options can bring benefits and rewards, yet will be unforgiving where the wrong choice is made.

Making that choice between turbines and engines, natural gas and diesel, was perhaps not so difficult for a consortium of horticultural growers on the Isle of Wight that has ordered 18 Jenbacher gas engines to meet the energy needs of their greenhouses.

In early 1999, the Wight Salads consortium awarded the contract to supply a complete combined heat and power (CHP) system with draw-in carbon dioxide (CO2) capability to Clarke Energy, the UK distributor of Jenbacher engines. Secured in conjunction with UK-based project developer Independent Energy, the contract was described by Jenbacher as “a major breakthrough for Jenbacher and Clarke Energy in the horticultural sector”, and represents the largest ever CHP order in Europe’s horticultural industry.

The 18 gas engines will supply six nursery sites with electricity and heat, while CO2 will be extracted from the exhaust gases to increase crop yield. Surplus electricity from the power plant will be sold to the local Isle of Wight grid and will also be exported to the mainland UK national grid. The first phase of the project started undergoing commissioning in July 1999.

A changing market

Autogeneration is by no means a new phenomenon; large industrial consumers have been meeting their own energy needs with on-site generation for years. However, driven by deregulation and the opportunities that this offers, autoproduction is now a feasible option for the smaller commercial users as well as the industrial giants.

According to UK consultants Datamonitor, energy price is the clear driver in this growing market. Yet in spite of the positive influence of liberalization in Europe and the USA on this market, uncertainty over how liberalization will proceed and its long-term impact on prices has caused uncertainty among potential autogenerators.

Nevertheless, autogeneration continues to grow and by far the most popular technology choice is combined heat and power, according to Datamonitor. And depending on the application, gas engines are becoming a popular alternative to gas turbines. According to Jenbacher, gas engines are steadily increasing their share in the high-speed engine sector (over 1000 r/min), and as the company said in a recent newsletter: “gas engines are very trendy”.

As for all potential industrial autoproducers, market liberalization brought Wight Salads the opportunity to generate extra revenues through the sale of surplus electricity to the grid. In addition, generating its own energy needs will allow the company to have control over its own energy costs.

Figure 2. Carbon dioxide from the engine exhaust gas is fed to the greenhouses to increase produce yields
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As an autoproducer, Wight Salads aims to both reduce its energy costs and increase the yield from its greenhouse produce. A CHP system was an obvious choice given the required heat load of the nurseries, and the use of natural gas as a fuel will enable the extraction of CO2 gas to increase crop yields, as well as keep emissions low. Wight Salads also required a technology capable of constant duty base load operation, high durability and longevity with low maintenance requirements. Gas engines therefore seemed a natural choice, particularly given that they would not require the installation of special foundations and so could be installed quickly.

Tried and tested

The Wight Salads consortium comprises Arreton Valley Nursery Ltd., the largest independent tomato grower in the UK, Bridgeguild Ltd., which produces approximately half of the UK cherry tomato crop, and Macketts Land Nursery, which concentrates on cucumber production. These companies all market their produce through Wight Salads, and with a total glass area of 67 acres between them, they hope that reduced energy costs and increased productivity will enable them to increase their competitiveness.

Figure 3. Gas engine’s are becoming a popular alternative to gas turbines
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This aim should be achievable: CO2 fertilization with CHP is a tried-and-tested technology pioneered by the Netherlands. Since 1987, horticultural grower Nutsbedrijf Westland NV has ordered several cogeneration modules from Jenbacher for its greenhouses. The company now has a fleet of 150 units in this type of application, of which Jenbacher has supplied over 30.

Green plants derive their energy from carbon dioxide in the atmosphere which they convert into sugars and carbohydrates using photosynthesis, a process which requires sunlight. Thus by increasing the CO2 concentration in greenhouses from the natural level of 350 ppm to 800-1000 ppm, crop productivity can rise by as much as 40 per cent. This enrichment is normally achieved by burning natural gas in CO2 ‘cannons’, but diesel engines can be easily adapted so that their exhaust gas provides the CO2. Around 0.2 kg of CO2 arises per kWh of fed energy, and depending on the gas/air ratio, the CO2 concentration in the exhaust is 5-6 Vol%.

Clarke Energy will supply the 18 engines as a complete factory-built package. The engines, together with their control systems, will be tested on test-beds at Jenbacher’s main factory in Austria before transport to the UK, enabling faster and more efficient commissioning.

An efficient option

The power plant will supply a total of 36 MW from 19 model 616 Jenbacher gas engines. Each cogeneration module has an electrical output of 1944 kW and a recoverable thermal output of 2048 kW. At standard rating conditions without CO2 draw-in, overall efficiency is 84.8 per cent and NOx emission are below 500 mg/Nm3 (at five per cent O2). The 16-cylinder engine has a V 60 configuration and operates at a nominal speed of 1500 r/min. Bore is 190 mm and stroke 220 mm. The dimensions of each module are 7 m by 2.1 m by 2.7 m.

Thermal energy from the generating units is collected from the intercooler, jacket water, lubrication oil and the exhaust gas by heat exchangers. Up to 50 per cent of the energy input can be recovered as heat, according to Jenbacher. The water temperature from the engine is 90°C and is 70°C in the return direction.

The exhaust gas from the generating modules is passed through a SCR catalytic converter to remove NOx gas and oxidise CO emissions. By spraying the exhaust gas with urea, the NOx content is lowered by more than 90 per cent. The NOx and CO levels of the ‘cleaned’ gas are constantly monitored by the plant’s control system, and the urea spraying and oxidation processes are regulated accordingly. In case of a failure in the exhaust cleaning plant, the exhaust gases are passed directly to a stack.

The CO2-rich gas is then passed through a two-step heat exchanger where its temperature is reduced to 50-55°C. It is then fed into the greenhouses via a plastic tube by means of a blower. The CO2 levels in the greenhouses are also monitored by the control system.

By using the heat, power and also the CO2 in the exhaust gas, overall efficiencies of the system rise to 95 per cent.

A growing market

Jenbacher sees the horticultural industry as a major growth area for CHP applications and its products. In June 1999, Clarke was awarded a contract for the supply of four Jenbacher gas engines totalling 10 MW to Valley Grown Salads in the UK. The engines are expected to be delivered before the end of 1999 and will be installed at the Langridge Nursery, an eight acre site growing cucumbers and peppers.