Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

agrotech.dk

Article. Energy in fokus - from Kyoto to Copenhagen. - AgroTech

The sun

as a potential source of energy

In the long term there are likely to be demands for CO2-friendly or perhaps

even CO2 neutral production. There is no simple method of solving this

problem. Several solutions have to be brought into play at the same time,

and exploiting the heat from the sun is a strong contender

Ole Skov oes@agrotech.dk, and Jens Rystedt jor@agrotech.dk, consultants, AgroTech

Solar radiation is potentially an enormous

source of energy, and in a greenhouse

the annual influx of energy is about

850 kWh/m2 (measured on a horizontal

surface inside the greenhouse); approximately

twice as much as the greenhouse’s

own energy consumption.

In principle, a greenhouse is one big

solar panel, and it will become increasingly

common to collect some of the

surplus energy from the greenhouse and

store it for later use. However, so far the

investment required has been too big to

make this possible. Storage of the large

amounts of energy for use in the winter is

a great challenge.

Perhaps better insulation of greenhouses

would be a good start, as this would

reduce the amount of energy which would

have to be stored. With better insulation, it

is not unrealistic to halve the energy need.

After this we can tackle exploitation of the

sun’s energy, either directly in the greenhouse,

or by using solar panels, or through

a combination of the two.

Harvest of solar energy

Great challenges have to be overcome

before we can meet all our needs using

solar energy: How is the energy to be harvested,

and how is it to be stored?

The plants seen so far, mostly in the

Netherlands, uses ventilation units to harvest

the energy by transferring it from the

air in the greenhouses to water, which is

often stored underground. This method

works at relatively low temperatures, especially

in the storage facilities. The energy

has to be transferred back to the greenhouse

from the water to the air.

The surplus energy from the sun can

be harvested and stored in many different

ways. The method to be chosen depends

on the existing equipment and on the

percentage of the energy consumption to

be covered initially by the surplus energy.

At many sites, buffer tanks and other

types of water storage have been installed

which can be used as energy stores for

short periods – e.g. from day to night or

over a few days. For example, if the surplus

energy is collected on a daily basis via a

ventilation plant in which the water temperature

is heated to 70°C and then stored

in a 500 m 2 buffer tank, this can be enough

to cover the energy consumption of a 5000

m 2 greenhouse for four to five summer

months. This corresponds to about 20 percent

of the annual energy consumption.

Water is either heated using traditional

solar panels, or using a heat pump.

New possibilities

So, even with a small heat storage facility,

it is possible to reduce the use of fossil

fuels. In combination with installation of

ventilation equipment, this provides new

opportunities to secure energy supplies,

and ventilation equipment in the greenhouse

means it is possible to control and

adjust the greenhouse climate by evening

out the differences in temperature and

humidity.

If we want to increase the percentage of

our energy which comes from solar heating,

we have to think in terms of larger storage

capacity. For example, underground

storage, or in heat storage ponds, which

are like large rainwater basins covered

with insulating material.

There is still some way to go before we

can say that we are independent of fossil

fuels, but energy directly from the sun is

certainly one of the possibilities we should

keep our eye on.


20 ENERGY IN FOCUS

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