1. Solar-thermal driven cooling of greenhouses for food production
World Renewable Energy Congress IX
Florence, August 2006
Solar-thermal driven cooling of greenhouses for food production
Philip Davies

2. Background
Many sunny, dry countries are also ones where population and demand for food is growing.
Production of food in greenhouses allows water conservation and greater yield than in open fields.
But in a hot country, a greenhouse often needs cooling (a need that will increase in future due to global warming).
To provide some background and explain the motivation, here are some basic well known facts …
(Can be read out more or less)
These facts lead straight on to the objectives of this research

3. Objectives
To use solar energy to provide cooling of greenhouses, on a scale useful for food production
Without using freshwater
Instead, to make use of seawater and its constituents – in particular salts in seawater having hygroscopic (ie. water-absorbing) properties
Which is to use solar energy to provide cooling of greenhouses (large greenhouses, not small ones like you might have in a garden!).
Note that this is quite challenging to the large amount of cooling required, which would be difficult to achieve using most concepts of solar-driven cooling.
Just as we would like to use solar energy and avoid using up precious energy resources from fossil fuels, we would also like to conserve water. So we propose to avoid using freshwater in our system – instead we will use Seawater and substances contained in it.
In particular we would like to make use of hygroscopic salts present in seawater.
Let me now explain briefly something about the composition of seawater

4. Tending to form hygroscopic chlorides
It is well known that seawater contains sodium chloride
But less well known that it contains a number of other salts – in particular salts of magnesium and calcium. Chlorides of magnesium and calcium are hygrcscopic as solids and as concentrated solutions.

5. Solar salt works bitterns salt
Let us now look at what happens in a solar salt works, where water is evaporated from seawater to make salts. As this evaporation takes place, the remaining liquid seawater becomes more and more concentrated, mainly in magnesium salts which give it a bitter taste. For this reason it is known as bitterns.
salt

6. Composition of bitterns vs. raw seawater
Based on Amdouni (2000)
Sfax saline S.E. Tunisia
The composition of such bitterns has been studies in detail. Here we see some results from the Sfax salts works in Tunisia.
Less hygroscopic
More hygroscopic

7. Hygroscopicity of bitterns solutions
(Preliminary results by G. Lichnos, PhD student)
In our lab, we made up solutions similar to those occuring salt works and confirmed that there are indeed hygroscopic, able to lower the humidity of air to below 35%.

8. Evaporative cooling (conventional system)
Water
GREENHOUSE
Ambient air
Cool air
So how would we use these hygroscopic solutions?
Let us first look at a conventional greenhouse cooling system, of the kind used today. These typically rely on evaporative cooling. As such, they are limited with regard to the amount of cooling that they can achieve. For the air to be cooled, it must absorb water, and it has a limited capacity to do so depending on its humidity.

9. Desiccation + Evaporative cooling
Liquid desiccant
e.g. bitterns
Seawater
GREENHOUSE
Ambient air
Cool air
I propose to modify such a system by first lowering the humidity of the air,
Air here dry – therefore easy to cool

10. Research carried out (1)
Literature search to compare:
Greenhouses with existing applications of liquid-desiccant cooling systems
Properties of seawater-derived liquid desiccant with more common types of liquid desiccant
So let me summarise briefly the work that we have carried out to date on this. Mostly this has been based on literature searching and modelling, as we are just starting to carry out experimental work.
Since cooling systems using liquid desiccants are not a new idea, we decided to compare our proposal with previous work. In particular we decided to contrast the application to greenhouse cooling to other applications – in particular to the application of cooling human dwellings.

11. Solar liquid-desiccant cooling: comparison with human dwellings
Greenhouse
Previous research
Significant
Little
Solar loadings
Low (50 W/m2)
High (300 W/m2)
Comfort limit
(wet bulb °C) 20
24 to 27
Heat removal needed per floor area
LOW
HIGH
Heat removal per kg of air exchange
In fact, most previous research has focussed on human dwellings.
Here I summarise the main characteristics of each of these applications.

12. Comparison of liquid desiccants
conventional
seawater derived
LiBr
LiCl
CaCl2
MgCl2
Equilibrium RH% 6 11 29 33
Abundance litres desiccant per m3 of seawater
0.004
0.003
2.3 13
Toxicity
Medium
Low
So what does this imply for the type of cesiccant that we should use?
To answer this, ;et’s look at the properties of desiccants – the more conventional type (such as lithium salts) and the seawater-derived type (calcium and magnesium salts).
Simplified from Davies & Knowles, Desalination, 2006.

13. Comparison of liquid desiccants
conventional
seawater derived
LiBr
LiCl
CaCl2
MgCl2
Equilibrium RH% 6 11 29 33
Abundance litres desiccant per m3 of seawater
0.004
0.003
2.3 13
Toxicity
Medium
Low
More hygroscopic
It is true that lithium salts have the advantage of being more hygroscopic, which would be important for a human dwelling, but for greenhouses that may be less important as we do not need to reach such low temperatures.
Then we have some important advantages of the calcium and magnesium chloride – abundance in seawater and low toxicity.
More abundant
Less toxic
Simplified from Davies & Knowles, Desalination, 2006.

14. Comparison of liquid desiccants
Suited to cooling of
greenhouses
Comparison of liquid desiccants
Suited to cooling of human dwellings
conventional
seawater derived
LiBr
LiCl
CaCl2
MgCl2
Equilibrium RH% 6 11 29 33
Abundance litres desiccant per m3 of seawater
0.004
0.003
2.3 13
Toxicity
Medium
Low
More hygroscopic
More abundant
Less toxic
Simplified from Davies & Knowles, Desalination, 2006.

15. Research carried out (2)
Modelling to predict cooling achievable with the proposed system

16. Additional cooling obtained
Air temperature delivered (deg.C) – average daily maximum
based on psychrometric chart (80% effectiveness)
Ambient
Evaporative
Desiccant
50% Equil. RH
We carried out some simple modelling to comoare the temperatures we could achieve in a cooled greenhouse, using the conventional evaporative system and the desiccant system.
We looked at different locations at different time of year
Typically, the system can achieve temperatures 5 degrees C lower than with the conventional, evaporative cooling system.
TUNIS
ABU DHABI
Month

17. Abu Dhabi: Extension of growing season
Davies, Solar Energy, 2005
This means that we can extend the growing seasons of crops. For example, for tomatoes and cucumbers in growing in AbuDhabi, we could extend the optimum growing season from 7 months of the year to all year round.

18. Implementation How would the whole system be implemented?
There are different ways but here is one example that I am proposing (explain)

19. Benefits
Achieve lower temperatures than with conventional cooling systems, providing longer growing seasons and greater range of crops.
Exploit by-products from desalination and salt production

20. Conclusions
Seawater contains hygroscopic salts of magnesium and calcium.
As concentrated solutions (bitterns), these salts can be used as liquid desiccants for cooling greenhouses
Sunlight would be used to generate and regenerate the desiccant.
Future work will focus on practical experiments to verify the modelling reported on today.
Emphasise that the solar energy would be used to concentrate the bitterns from seawater and to keep them concentrated.