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ABOUT ENERGY

By Nicolas Ancion (ULg), David Volk (EBF), Ismaël Baraud (CSTB), Laurent Reynier (CSTB), Bernard de Gouvello (CSTB), Marcel Deravet (IFSB), Nicolas Brulard (Gally)

Finding a good system to heat the greenhouse in winter (to avoid winter frost)

A good system to heat the greenhouse in winter is based on two important aspects:

  • Getting sufficient energy into the greenhouse

  • Avoiding energy losses.

The first aspect is in line with the main goal of the greenhouse – plant production. The primary energy source is sunlight, which is mainly influenced by:

  • The greenhouse design and orientation

  • The covering material

  • Obstacles in the surrounding area

Since solar intensity is low in winter, further energy sources from commodities should be included:

  • Waste heat capture from residential and office heating

  • Industrial waste heat sources

  • Additional thermal solar energy with hot water storage

  • Waste heat from the household equipment like air conditioners and ventilation.

All the above-mentioned aspects might be enough to keep out the winter frost but are often accompanied by a high installation cost and/or operation cost. Therefore, avoiding energy losses might be an opportunity to investigate:

  • Sidewall perimeter insulation, especially important for walls facing north, where no sunlight can be captured

  • Removable transparent insulation: while having decreased transmissive properties, they can be used to keep the heat inside, especially if there is no winter production and not all light is needed

  • Heat-saving devices (heat shields) in the greenhouse, which are standard techniques but might turn out to be costly too.

An additional measure, one can also reduce the heated space in the greenhouse and adapt it to production; two significant possibilities can be investigated:

  • Only heat the water used for hydroponics to keep the plants warm while not heating the greenhouse (crop-dependent)

  • Used fleeces or plastic tunnels over the crops to further reduce the heated space

All above-mentioned techniques can be combined in an optimal way to increase efficiency while keeping costs low.

How to enrich the atmosphere of greenhouses with CO² and how to succeed in doing so

Enriching the greenhouse atmosphere in CO² can be a good opportunity to increase efficiency in a greenhouse environment. Plants use CO2 in combination with sunlight to grow, while emitting oxygen. With more CO2 in the air, crops can improve their growth. But they also need more sunlight to use the additional CO2.

Here are possible enrichment techniques:

  • Using passive CO2 enrichment techniques, such as ventilation of exhaust gas from an office building or other sources

  • Using the chimneys of the heat-generating facilities already installed in the building or needed for the greenhouse, as well as possible other sources surrounding the greenhouse

  • Using active systems that primarily rely on the combustion of fossil fuels

In most cases, exhaust gases can be directly used in a greenhouse without further treatment. Passive systems are generally preferred because they do not use valuable resources and do not generate further emissions.

CO2 can be generated by heating the greenhouse. This key aspect creates a synergy between heating the greenhouse and enriching its air in CO2. One final remark is that CO2 enrichment is often at odds with ventilation needs. In summer, when the benefit of high sun intensity and CO2 enrichment can have the highest synergies, the greenhouse is also likely to overheat, which may in turn damage the crops. If no active cooling system is used, ventilation is key, but also reduces enrichment. Therefore, there has to be a trade-off in a greenhouse between these two aspects, especially if enrichment is active and resources are used.

How to combine PV panels or solar panels with an RTG

Finding a way to implement PV systems along with an RTG can be helpful to maximise the use of energy from incoming sunlight. Several techniques are possible, but it is important to make sure that the plants in the greenhouse are prioritised for sunlight exposure, otherwise plant growth is sacrificed for energy production. On the other hand, during most of the summer days the maximum amount of sunlight exceeds plants requirements by far and creates a hot climate in the greenhouse that requires shadowing. Using the PV system for shadowing helps to cool the greenhouse and also usually generates enough electricity to maintain a self-sufficient operation of the greenhouse systems. Therefore, an adaptable system is necessary which does not impede plant growth.
New photovoltaic panels are being developed, that would allow the light waves necessary for plant growth to pass through while producing electricity.

How to manage condensate water in the greenhouse

Plant growth and high solar intensity cause the greenhouse air to accumulate humidity that condensates overnight when the air cools down. This leads to cold water dripping on the plants, making them vulnerable to pests and fungal infections. Additionally, this can lead to further contamination when outside air mixes with inside air and pollutants are transferred to humid air.

Measures can be taken to prevent condensation:

  • Ventilation reduces the amount of humid air and adds fresh and drier outside air. The disadvantage is that a lot of the water in the air comes from plant watering; this water is lost, and this increases the resource demand of the greenhouse

  • Cooling devices in which the water condensates and can be reused directly can be used. This implies a high energy demand

  • It might also be feasible to circulate the air through several colder areas in the bearing building, with designated condensate reservoirs (see Chapter II.7).

Since condensation cannot be prevented, some greenhouse builders use specifically designed heat bridges to generate condensate at certain spots where it can be managed more easily and reused.

Condensation is especially problematic early in the morning, and strong ventilation currently usually solves the problem. It can be useful, but it also implies a great energy loss.

How to optimise the greenhouse cover for optimal production in summer

A heat shield can be installed in addition to the already mentioned PV system to operate a greenhouse in summer. A heat shield is a moveable screen consisting of a material that blocks the sunlight partially or totally, leading to a reduced cooling demand. This also leads to reduced available light for the plants, which might result in reduced plant growth. It should be noted that plants get more than enough sunlight to grow in most cases and places in summer. Excessive light stresses the plants and reduces their ability to grow.

Potential benefit from installing an RTG on a car park

Using a car park that is not in operation anymore can have some benefits. First and foremost, finding a new way of operating a disused sealed land has a lot of benefits for the town itself. Additionally, car parks are designed to bear high loads, they can usually bear a greenhouse without any necessary reinforcement of their rooftop. Moreover, roof accessibility is generally a lot better than it is for comparable projects (the usual logistics problems of RTGs are mitigated). On the other hand, some of the usual benefits of a rooftop are missing:

  • A carpark is usually not heated, therefore the waste heat potential is absent

  • Quite often the different levels are not closed so that the greenhouse will be ventilated from underneath, increasing transmission losses

Furthermore, the former carpark might bring uncontrollable contamination into the greenhouse.

Use of a water tank in the greenhouse to promote thermal inertia; how to do so

In principle, this is possible, but implies restrictions mainly regarding the weight and size of the tank. It is important to check the bearing capacities of the building. Additionally, RTGs can be limited in size, so a larger water tank can take up important growing space.
More generally, it is more appropriate to place the tank on the ground or underground when harvesting rainwater, even if this leads to an energy demand for the pumps.

How to improve the energy efficiency of the host building or RTG through the RTG design

This question is heavily influenced by the project and needs thorough examination in each case. However, basic possibilities can be checked if possible. Here are a few outlines that add to the previously mentioned answers:
1. If some of the greenhouse surfaces face a direction in which there is no sunlight coming in (north), these surfaces can be strongly insulated. The heat from the building, even insulated, will limit losses
2. If walls of the existing building are available, using them will decrease the surface of the greenhouse and reduce transmission heat losses
3. If there is any kind of energy waste on site, using it can be helpful to increase energy efficiency
4. Trying to cover the greatest possible roof area with the RTG will also reduce transmission heat losses from the building
For example, energy efficiency was improved by 13% in Gembloux by adopting a lean-to design. Nevertheless, each project is different and needs to be assessed.