Do thermal screens save energy and give better CO2 levels?
The use of thermal screens has become common place on many edible crop nurseries. They've been shown to save large amount of energy, paying back their cost quickly. In 2005, HDC project PC 198 showed that a thermal screen could save 100 kWh/m2 p.a. of gas with quite conservative energy saving set points.
But of course, reduced energy use can mean that there is less CO2 for enrichment. In 2005 we believed this wasn't critical as winter screen use coincided with the time when CO2 demand was low and vents are closed. And of course higher summer CO2 demand occured at a time when thermal screens were not being used.
However, now, with extended screen use as growers strive to save even more energy, we see extending use of thermal screens into mid to late April. So do will we finally see reduced CO2 levels as a result? The answer is no. In fact in most cases the opposite can actually be true – this article explains why, using our Focus Nursery at Wight Salads as the example.
Greenhouse heat demand
The graph below shows the pipe rail temperature in one of the greenhouses at Wight Salad’s Lane End Nursery without a thermal screen.
(Click on graph to see a larger version)
Note that the pipe rail heating system operates at 70 degrees C for most of the night. In fact, most hot water transport systems are set to be at least 5 degrees C higher than this (75 degrees C in this case). This means that as soon as the heat store temperature dips below 75 degrees C the boilers have to turn on to support temperature – in the example above the boilers turned on at 02:00.
How does this affect CO2 levels?
To gain the greatest benefit from a heat store it needs to be as cold as possible by sunrise. In the above example the heat store was 65 degrees C. Ideally you would hope to get it down to at least 55 degrees C or less. A heat store temperature at 65 deg C leaves you with with 28% less CO2 enrichment capacity (maximum heat store temperature of 90 degrees C) than ideal.
What does this cost?
Taking the rule of thumb figure of 200 m3 of heat storage per hectare; a 10 degrees C reduction in storage capacity is equivalent to having to burn an extra 2,700 kWh of gas/Ha. If CO2 enrichment is a major concern, the options are:
- Burn the gas anyway and ‘throw away’ the heat. Cost - £41/Ha/day
- Use pure CO2 instead, you would need 0.5 tonnes of pure CO2/Ha. Cost - £60/Ha/day
How do thermal screens help?
Thermal screens reduce the heating pipe temperature requirement by 10-15 degrees C. In the example above, the greenhouse pipe rail temperature could be reduced to around 60 degrees C. This lower temperature would allows more heat to be removed from the heat store, cutting back on night-time boiler use but allowing a greater potential for daytime use when CO2 is required.
However, note that the heat store / boilers are controlled by the ‘worst’ greenhouse (the one with the higest heat demand). So even if only one of your greenhouses requires 70 degrees C that is what the heat store / boilers have to deliver.
The greenhouse mentioned earlier is an older wide-span type and some people claim that it isn’t worth installing a moveable thermal screen in them. A previous focus nursery (Eric Wall Ltd) did believe it was worthwhile, and have just completed their second winter with one. Installation in a wide span house is more difficult and therefore expensive (budget around £8/m2).
However, at a practical level it has performed well and even at the relatively low gas price of 1.5 p/kWh (45 p/therm) the pay back works out at just over five years based on 100 kWh/m2 saved. With increasing gas prices we estimate that the more ‘aggressive’ energy saving set points used by most growers nowadays will increase savings to as much as 120 kWh/m2 . So pay backs of four years are possible. With some CO2 related benefits, installing thermal screens in some older greenhouses could be quite an attractive investment.