CMW Winter News 2009

12% Heat saving in year 2 of HDC project PC 278 at CMW customer Mill Nurseries

Fig 1. Showing the polythene Optimiser air duct suspended beneath the hanging gutter

PC278 was commissioned by HDC as a 3 year project starting in the 2008 season. The purpose was to investigate what benefits might result from a mechanical ventilation system drawing a controlled amount of outside air into the glasshouse. FEC services were appointed by HDC to design and supervise the trial and a Priva Optimiser air handling system was supplied by CMW for the one hectare block at Mill Nurseries in Keyingham at the beginning of 2008.

Mill Nurseries is one of the leading tomato producers in the UK. They have been involved in HDC projects before and were chosen for their expertise and because they could provide two identical blocks for comparison as well as being willing to make the considerable capital investment required.

The trial consists of a one hectare block fitted with the Priva Optimisers adjacent to an identical block with conventional ventilation for comparison. The Optimiser block is fitted with Priva air handling units at 8 row spacing (12.8m) giving 2 complete air changes per hour (108,000 m³/hr). Each Priva Optimiser is fitted with a heat exchanger connected to a low temperature water circuit (max 50C) which provides the glasshouse with a maximum heat capacity of 45W/m² (450kW). This can be supplemented by additional heat from the boiler through the conventional heating pipes when required.

 At the TGA conference in Coventry a few weeks ago Tim Pratt reported on progress over the first two seasons.

'Not without it's challenges'  - Tim Pratt

‘The project has not been without its challenges’ says Tim Pratt ‘We encountered a serious and totally unexpected temperature uniformity problem in 2009.’ FEC’s monitoring probes revealed a 2C temperature variation from the gable to the centre path in the trial block compared with 1C in the conventional block next door. After much head scratching FEC and Priva’s air handling specialists came up with a modification which may have cracked the problem. (Trials in the colder weather since the TGA confirm that progress has been made.)

‘Our second biggest challenge has been learning how to use the new control facilities the system provides’ Tim told the TGA delegates. For example, in respect of heating Tim asks: ‘Is it better to give priority to the heated air from the Optimisers or the conventional pipes?’ and when cooling is required ‘Is it better to use the outside air through the ducts at the base of the crop or at the top?’ Similar decisions arise when the critical issue of humidity is being considered. The trial has underlined the notion that the grower will have new choices about how to achieve the ideal climate.

'This system can give me much more control'  - Tony Mills

Tony Mills, the grower for this trial, has noticed subtle differences in plant development. ‘The Optimiser crop has tended to be more generative’ says Tony. What is remarkable is that this extra generative growth has been observed even though Tony has been able to run lower pipe temperatures and lower HD’s in the Optimiser crop. Tony is quite excited by the potential of the system. ‘It hasn’t been easy to get this far’ says Tony ‘But I can see that the new techniques we’re developing give me much more control of the plant’.

Impressive yield increase

All this is academic of course unless the new technique delivers an acceptable return on investment. That won’t be known for sure until the end of the trial next year, but already there are favourable indications. The Optimiser block used 12% less heat this year than the conventional block and yielded a whopping 7.6% more. The energy savings alone don’t so far justify the investment, but when extra yield is taken into account, it could be a different story. And Tim is convinced that one of the biggest advantages of this technique is to make use of low grade heat from CHP or 3^rd party waste heat providers.

‘Despite the problems, we’ve been able to demonstrate significant potential in this trial’’ says Tim Pratt. ‘The project has a year to run and in the final year we aim to use less electricity and save even more heat’

The end of season yield performance was particularly impressive. In view of the technical problems the trial encountered, it's not too difficult to imagine that there could be more to come from this trial. Roll on year three!

Acknowledgements Thanks to Tim Pratt of FEC and Tony Mills of Mill Nurseries for their help in the preparation of this article. PC278 is an HDC funded project. Grateful thanks also to HDC for permission to publish this news item.

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Fig 2. Showing the main filter, heat exchanger and Optimiser fan

Fig 3. Total yield in the Optimiser Block was 7.6% higher than the conventional block by the end of the crop