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UK’s biggest greenhouse places electrical reliability high on the agenda

Peter Duncan, managing director of Cressall Resistors, explains how the NERs (Neutral Earthing Resistors) the company has provided are helping to guarantee that the Thanet Earth greenhouses can continue to produce crops 52 weeks a year

UK’s biggest greenhouse places electrical reliability high on the agenda

Generating electricity and selling it back to the grid is both a 21st century urban utopia and the goal of countless businesses that invest in combined heat and power (CHP) today. However, there can be few companies where the primary goal is to utilise both the waste heat and the carbon dioxide produced by CHP, but that is the case at Kent-based Thanet Earth – the UK’s largest greenhouse complex.

Thanet Earth is an £80M project consisting of seven high-tech greenhouses; each one about the size of ten football pitches, totalling 91 hectares. Each greenhouse grows its crops hydroponically, which means the salad vegetables it produces are grown in an inert medium – not soil. It’s also an engineering marvel that features an automated packaging plant, rainwater recycling and horticultural research as well as CHP.

Based on the Isle of Thanet in Kent, once completed, the greenhouses will generate a 15% increase in the UK’s salad vegetable production. Operational since the beginning of 2009, the three greenhouses already constructed are currently producing a staggering two and a half million tomatoes a week, 52 weeks a year, as well as around 750,000 peppers and over 500,000 cucumbers, picked weekly during the shorter season between February and October.

Thanet Earth is owned by a group of salad growing companies, including the UK's largest privately-owned fresh produce supplier, the Fresca Group, which has a 50% stake in the business. The remaining 50% of the company is owned by three other salad growing specialists – Kaaij Redstar, Rainbow Growers and A&A.

An independent study by Bidwells Agribusiness reveals that the use of combined heat and power actually contributes a negative carbon emission towards the total measured Thanet Earth carbon footprint. The greenhouses themselves require very little operational power so almost all electricity is exported through a newly created link to the National Grid. Once fully operational, this will provide enough power for an incredible 50,000 homes, about half of those in the Thanet area.

Each of the 140m long greenhouses are fed from a group of on-site generators. The tomato glasshouse which features lighting to assist the plant growth in the winter months is kept energy efficient using a system of curtains and shades to keep at least 95% of the heat and light inside. This also avoids light pollution, which is one of the aesthetic objections regularly raised by anti-greenhouse campaigners. Where lights are used, in winter tomato production, the power is also generated on site. The carbon dioxide produced by the heating process is used to feed the plant’s growth.

When the level of production is as high as it is at Thanet Earth, every crop represents a huge investment on the part of the companies who own the business. To illustrate the scale of this investment, seven day’s production at Thanet Earth represents 13.4% of all the salad vegetables grown commercially in the UK that week. Losing that would certainly be a crippling financial blow to the host of companies involved in the production, transport, processing and retailing of the produce. As a result, the reliability of the combined heat and power is of paramount importance to the greenhouses. An essential part of ensuring reliability for the CHP system is having an effective NER set-up, to limit the current, which would flow through the neutral point of the generator in the event of an earth fault. In doing so, the NER limits the fault current to a point that would not damage the generator, allowing it to be safely shut down while the other generators continue supplying the essential heat and carbon dioxide to the plants being grown on the site. The alternative could mean damaged generators, which could take days or weeks to replace or repair, costing six figures in lost production.

Cressall manufactures neutral earthing resistors for system voltages up to 132kV and for any current and time rating. Its NERs are a key component of the earth fault protection scheme in the majority of medium voltage electricity generation and distribution systems, including those belonging to major energy companies such as Central Networks, EDF and Yorkshire Electricity.
Cressall NERs have only a minimal and predictable alteration in resistance during operation. As a result, the protection levels can be accurately pre-determined. Further predictability comes from the fact that, in contrast with an earthing reactor, a Cressall NER does not induce unpredictable phase changes or resonances into the fault current and so does not require matching to the associated transformer. This makes integration a relatively easy process.

Integration is further simplified because Cressall NERs are compact and do not require site calibration or auxiliary power supplies. Maintenance can be limited to periodic inspection and cleaning only and anti-frost and anti-condensation heaters are not required – the latter function would be useful in a greenhouse while the former is more useful when the unit is sited outside
Edwina Kilford, managing director at Thanet Earth Marketing, said, "A consistent and reliable supply of heat and carbon dioxide is essential to meet the promises we make to our customers.

And keeping those promises is our top priority every day. As a result, the CHP system we have installed is more than just an environmentally-friendly way of supplying heat and light – it’s an essential component in our business model. The work Cressall has done in ensuring the reliability of this system has been very effective and very much appreciated."

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