COMPARISON OF GREENHOUSE HEATING WITH
GEOTHERMAL ENERGY, SOLAR ENERGY AND BIOMASS
in the conference “Geothermal energy applications in agriculture “ Athens,
3-4 May 2004 .
J.Vourdoubas
TEI of Crete, Greece, Department of Natural resources and Environment
gboyrd@tee.gr
ABSTRACT
Low enthalpy geothermal energy finds many applications in heating
greenhouses where it can cover all their heating needs. Apart from the
geothermal energy, the solar energy and the biomass find applications in
Greece and in other countries for greenhouse heating.
However although the geothermal energy and the biomass can cover all the
heating needs of the greenhouses, the solar energy can cover them only
partly.
In this paper a comparison of these renewable energy sources for heating
greenhouses is presented, as well as a valuation of them with various criteria.
1.INTRODUCTION
Renewable energy sources find many applications in agriculture and
among them in heating greenhouses.
Geothermal energy, solar energy and biomass have been used in Greece and
abroad for this purpose, (Table 1).
 Solar energy is abundant in Greece and there are various ways that
this energy source can be used for heating greenhouses.
 Biomass also, particularly olive Kernel wood has been used in the
greenhouse heating.
 Low enthalpy geothermal energy, has also been used extensively in
heating greenhouses.
However the application of these renewable energy sources, has various
environmental implications. There fore various, antipollution actions should be
taken, when these energy sources are used for heat generation in the
greenhouses. At the same time they have various advantages in comparison
with the conventional energy sources and probably they will find more
applications in this field in the future. In table 2, the CO2 emissions which are
avoided when the greenhouse Is heated with RES, are presented.
2.GREENHOUSE HEATING WITH GEOTHERMAL ENERGY
Heating greenhouses with low enthalpy geothermal energy,
(temperatures below 100 oC) is rather easy.
The geothermal fluid can be transported from the well to the greenhouse,
with insulated tubes and can be circulated either directly inside the
greenhouse in plastic tubes, or indirectly via a heat exchanger.
Various problems arise when the geothermal fluid is polluted.
In this case the pollutants should be removed before the use of the
geothermal fluid. Usually the energy of the fluid can cover all the heating
needs of the greenhouse with low cost that is very usefull for the greenhouse
owner. Therefore the geothermal energy can substitute the oil or the natural
gas as a fuel for greenhouse heating.
Sometimes geothermal energy can be combined with heat pumps for
greenhouse heating. In this case the heat pump rises the temperature of the
fluid to 50-65 oC, so it can heat the greenhouse more easily.
It is estimated that to day 15 hectares of greenhouses are heated with
geothermal fluids in Greece, mainly in Aegean Islands, in Kyklades and in
Northern Greece,(Table 3).
After its use the geothermal fluid is usually disposed underground inside a
new well, which Is located In some distance from the old one.
table 1:Methods of greenhouse heating with geothermal energy, solar energy and biomass.
RENEWABLE ENERGY SOURCE
1. SOLID BIOMASS
(like olive kernel wood e.t.c)
2. LOW ENTHALPY GEOTHERMAL
ENERGY
3. SOLAR ENERGY
HEATING SYSTEM
- Heating with warm water produced
with the biomass burning
- Direct heating or heating via a heat
exchanger
- Heating with the geothermal fluid
and a heat pump
- Active solar systems heating air or
water
- Passive solar systems like
* underground heat exchanger
* plastic bags filled with water Inside
the greenhouse
* Trombe wall
table 2:Greenhouse heating and co2 emissions
Heating fuel
Oil
CO2 emissions
(tn / hectare · year)
402
*Heating needs 1.000.000 kcal/h · hectare
* Heating period 1200 h/year
table 3:Geothermal greenhouses in Greece (1995)
DISTRICT
NORTHERN GREECE
LESVOS ISLAND
KYKLADES ISLANDS
TOTAL
AREA
(hectares)
10,03
3,75
1,30
15,08
SOURCE (1)
3.GREENHOUSE HEATING WITH SOLID BIOMASS
Biomass either solid or biogas, can also be used for greenhouse
heating. Solid biomass particularly olive Kernel wood or agricultural residues,
find various applications in heating greenhouses. In this case the solid
biomass is burnt and heats water which is circulated in tubes inside the
greenhouse.
In a different case, the gases from the biomass burning are circulated
in large, plastic tubes and heat the air Inside the greenhouse.
Olive Kernel wood is abundant in olive oil producing areas, it is easily handled
and its heating value is around 3500-4000 Kcal/Kg. Also its price in relation
with its heating value is rather low in comparison with the oil. Therefore it
consists a good raw material for greenhouse heating.
It is estimated that in Crete for the annual heating of a greenhouse, 1000m2,
30-40tn of olive kernel wood are needed.
The heating systems that use olive kernel wood are constructed from
local small industries which are specialized in these systems. However
sometimes various pollution problems are created with the use of olive kernel
wood, due to the fact that the burnt gases are not cleaned. Therefore
attention should be paid to this point and antipollution water filters and
cyclones should be installed with the olive kernel wood heating system. Apart
from the olive kernel wood, various other agricultural residues can be used for
greenhouse heating. Also the gaseous biomass like biogas (rich in CH4) can
be used for this purpose.
The use of the olive kernel wood like geothermal energy, can cover all the
heating needs of greenhouses.
This fact together with the low cost of the olive kernel wood in
comparison with the oil, makes these systems very attractive in heating
greenhouses.
4.GREENHOUSE HEATING WITH SOLAR ENERGY
The solar energy can be used with various ways for heating greenhouses.
There are two kinds of solar heating systems.
a) The Passive solar systems
b) The Active solar systems
The solar heating systems can only cover part of the heating needs of the
greenhouses. Also they are used when the desired temperature inside the
greenhouse should be only few centigrades above the ambient temperature.
Sometimes together with the solar heating system we are using another
heating system, to cover all the heating needs of the greenhouse.
 The main passive solar systems for greenhouse heating are. Passive
solar system with heat storage in plastic bags filled with water inside
the greenhouse.
 Passive solar system with heat storage in gravel underneath the
greenhouse.
 Passive solar system with heat storage in a concrete wall (TROMBE
WALL).
The main active solar system for greenhouse heating are.
 Active solar system with normal solar collectors.
 Active solar systems with plastic solar collectors.
 Heating systems with solar ponds (only experimentally ).
The abovementioned solar heating systems are usually covering only part
(10-60 %) of the heating needs of the greenhouses, obtaining temperatures
inside the greenhouse, 1-15 oC above the minimum outside temperature.
5.EVALUATION OF THESE METHODS FOR HEATING GREENHOUSES
Various parameters can be examined for the evaluation of the
abovementioned methods for heating greenhouses.
 When the CO2 emmissions are considered the solar and the
geothermal energy do not create such emissions like biomass.
However biomass has neutral impact to the greenhouse effect.
 Considering other environmental implications, geothermal energy is
likely to has higher environmental implications than solar energy and
biomass.
 When the covering of the heating needs of the greenhouse is
considered, geothermal energy and biomass can cover all the heating
needs but the solar energy only part of them.
 When the degree of Automation of the heating system is considered,
geothermal energy and biomass give better results than solar energy.
 If the manufacturing cost of the heating system is considered the cost
varies depending on the application.
If the operational cost of the heating system is considered, then the
operational cost when biomass is used, is higher than the case when solar
and geothermal energy are used. However in any case the operational cost is
lower than using oil or gas as a fuel.
 When the availability of the energy source is considered, biomass, solar
and geothermal energy are local renewable energy sources, in
comparison with the oil which is imported (in Greece ).
 If we consider the possibility of the local manufacturing of the heating
systems, in all cases the heating systems can be manufactured
locally,usually by SMEs.
 If we consider the heating fuel as a resource, then the solar energy,
the biomass and the geothermal energy are renewable resources, in
comparison with the oil or the gas which are non-renewable resources.
6.CONCLUSIONS
1. Low enthalpy Energy can be used for heating greenhouses where it
can cover all the heating needs of the greenhouse.
Depending on the dissolved substances in the geothermal fluid, various
environmental implications appear when the geothermal energy is used.
2. From the other renewable energy sources, solar energy and biomass
can be used for heating greenhouses. Although biomass can cover all
their heating needs, the solar energy can cover them only partly. As far
as the environmental implications are concerned, the solar energy
creates less pollution problems than geothermal energy and biomass.
3. There are various applications of these renewable energy sources in
heating greenhouses in Greece and abroad. In all cases, the cost of
using geothermal energy, solar energy and biomass for greenhouse
heating is lower than the cost of using conventional fuels like oil or
natural gas.
4. It is expected that in the near future the R.E.S. and of course the
geothermal energy will find more applications in agriculture and in heating
greenhouses.
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