Is minimum energy use always best?

When growing greenhouse crops, it’s sometimes desirable to use a little more heat than strictly necessary to maintain adequate growing conditions. The purpose of this is to keep conditions comfortably away from those which could, for instance, trigger a disease problem. It’s like walking near the coast — but not too close to the cliff edge. 

This article shows how one of the GrowSave Focus Nurseries (Eric Wall Ltd) chose to use more energy than the bare minimum to guarantee good disease control and promote crop growth.

Using more pipe heat than necessary

The climate graph below shows the basic control settings, pipe heat inputs and achieved greenhouse environment during one week at Eric Wall Ltd.

The graph shows a minimum pipe temperature setting of 40°C throughout the day. Even when the greenhouse temperature was above the heating temperature setting and the humidity is acceptable, the minimum pipe temperature of 40°C still prevailed. This is most prominent before 20:00. This being the case, it could be argued that heating use is greater than it need be to maintain acceptable growing conditions.

Why did Eric Wall Ltd use this strategy?

Eric Wall Ltd was buying gas on a ‘day-ahead’ contract. During the week shown above, they were paying around 1 p/kWh. They considered this to be relatively cheap in comparison with gas costs at the time.

Grower Manager Richard Kooijman made the decision to increase the minimum pipe temperature setting to 40°C for the following reasons:

• Running a higher than necessary pipe temperature would help to minimise disease problems, which he hoped would pay dividends later in the season. Also, any additional costs associated with the application of fungicide sprays would be reduced or even eliminated.
• By running a higher pipe temperature and consuming more gas, it would be possible to achieve higher CO₂ enrichment levels in the greenhouse, which would increase yields.

Richard also argued that a minimum pipe temperature setting of 40°C was still not that excessive, especially when compared to what growers used a few years previously, when settings of 50 to 60°C were common.

What did this strategy cost?

A close look at the data from Eric Wall Ltd shows that, over a typical day in the week shown above, the minimum pipe temperature setting was:  

40°C higher than necessary for 6 hours
and
5°C higher than necessary for 13 hours

With an assumed boiler efficiency of 85%, the elevated minimum pipe temperature consumed an additional  8,000 kWh/Ha/day. Costing £80/Ha/day or £560/Ha/week. As a bonus, burning this ‘extra’ gas produced 10.6 Tonnes of CO₂.

If this extra heat had no other benefits — such as improved disease control — CO₂ would have cost £52/Tonne. This compares favourably with the cost of pure CO₂, which even without surcharges and storage costs is typically around £100/Tonne.

And was it worth the cost? Well this has to be weighed against any reduction in disease risk and the yield benefit of the extra CO₂ produced. This is a judgement that only the grower can make.