Solar thermal energy to support the heating of greenhouses in the ...

Solar thermal energy to support the heating of greenhouses in the
southeastern Spanish
José Mª Cámara-Zapata
Department of Physic and Computers Architecture, Miguel Hernandez University, Ctra de
Beniel, km 3,8 sn, 03312 Orihuela, Alicante, Spain
E-mail: jm.camara@umh.es
Abstract
Spain is one of the leading producers of vegetables worldwide and between production
systems, greenhouse cultivation is one of the most prominent, both surface and production
due to high yields. This paper analyzes the technical and economic feasibility of
implementing solar thermal energy for heating greenhouses. We evaluate the energy needs
of the most important crops under cover in this area and discuss alternatives to the use of
solar energy to support the greenhouse heating. The results show that the simulated
systems can be economically competitive with all of the fuel normally used for these
applications.
Key words: solar energy collectors, heating greenhouses
1. Development and present state of greenhouse production in Southeast Spain
The geographical position of Spain, with proximity to European markets, and favourable
weather in the Mediterranean regions can be noted between the characteristics that have
contributed to a great Spanish horticulture development. From the 70, with the growth of the
plastics industry and its applications to agriculture, there has been a structural transformation
and economic horticultural products truly spectacular, especially in Almeria, Murcia and the
rest of the Mediterranean provinces.
Horticulture has a major role in Spanish agriculture, which stands for the quantity and value
of exports. Although at national level are important exports for some outdoor horticultural
species such as lettuce, cauliflower and cabbage, the largest share comes from greenhouse
cultivation of eggplant, zucchini, melon, cucumber, pepper, watermelon and tomato.
Considering the three major production areas of Spain, which are the provinces of Almeria,
Murcia and Alicante, it is found that with a cultivated area under irrigation rather small (just
over 50,000 ha), a cumulative production of approximately 3,500,000 t. Over 80% of the
irrigated area protected cultivation is practiced, mostly in greenhouse, and the rest is planted
outdoors. In the province of Almeria there is a greenhouse area near the 30,000 ha,
producing more than half of the fruits and vegetables of all Andalusia, with a final agricultural
production stabilized in the range above 2,000 million euros a year and an ancillary industry
very competitive in international markets (García Martínez, 2009).
In short, after nearly four decades of intensive production expansion in the Spanish
horticultural sector, the greenhouse is a key factor in competitiveness. One of the main
explanations for this situation is in production at lower cost than in protected horticulture
located north of the Pyrenees, due to the different energy dependence. For example, energy
consumption in greenhouse production represents 7% of the national total and 79% of total
agricultural in the Netherlands (Aramyan et al. 2007).
However, the growth and strength over the past four decades should not imply that it is a
stationary position. Between 1990 and 2004 the number of exports nearly tripled, in the
following years there was some stabilization, and has finally shown a significant drop in
production as a result of the economic crisis that has caused many owners have decided not

to grow in their greenhouses. With reference to tomato and pepper, both greenhouse
vegetable productions most important, the analysis of average prices received by farmers
shows a downward trend, higher for tomato than in pepper. Accordingly, it detects a loss of
purchasing power to the producer so it is necessary to pay attention on new strategies
(MAPA, 2012).
Fernández-Zamudio y Caballero (2006) conducted an economic study on greenhouse
pepper production technology with four levels representing the greenhouses of El Pilar de la
Horadada (Alicante), from the traditional (growing in soil without heating) to the most gifted of
technology (soil cultivation heating and carbon fertilization). In turn, each technological level
is assigned a height and type of structure with a whole set of complementary elements.
These features determine the type or variety of peppers to produce and therefore the quality
objectives and the markets they are trying to reach. The conclusion was that at the time of
the study all levels analyzed showed an acceptable return. The demands of heating and the
advantages provided by its use grow with the incorporation of technology into the
greenhouse. The use of heating has significant limitations because of its enormous cost
implications, especially with the current trend in fuel prices. However, it is an element that
significantly improves the quality and timing of collection, and therefore, the final sale price.
Therefore, its use at higher levels requires technical and economic control strict.
Spain has a good development of commercial structures but needs to increase its level of
technology to achieve higher levels of productivity and better quality. At the same time has to
comply with the requirements of quality and food safety, traceability marked by, and take
account of environmental standards. The developments in horticulture Spain seems to
indicate difficulties arising from increased competition from other Mediterranean countries,
like at the time, Spain became a threat to horticulture in central Europe, accounting a rate
significant quantities of absorbed by the markets (García Martínez, 2009).
2. Energy saving in greenhouses in the Spanish Mediterranean
Reduction in energy consumption lowers production costs and facilitate compliance with the
restrictive environmental and energy regulations affecting the sector, so that production is
more environmentally friendly and more competitive in the European market.
In a greenhouse you can save energy by improving the structures and heating systems. In
general, the structural changes reduce infiltration and increase the thermal insulation of the
greenhouse, either permanently or only during the coldest periods. Modifications of heating
systems try to optimize the recovery of heat energy from the burner and the heat supply to
plant more efficient. There are other aspects that influence, most notably the decline in the
percentage of the total front side surfaces and the roof cover.
Thermal roofing materials must be used, ie as impermeable as possible to the long wave
infrared radiation. The energy conservation techniques such greenhouse construction of
double-walled inflated, use of thermal screens, in addition to reducing heat loss and increase
the temperature of greenhouse other changes in microclimate, such as decreasing the rate
of ventilation, with generally negative effects. So the installation should take into account
these problems and provide the way to solve them.
Regarding the need to address sustainability issues in the production process, Montero et al.
(2008) describe how to reduce the environmental impact of properly using the equipment
elements and drivers of climate, the optimal application of CO 2 and what might be the best
use of the heating.
Last but not least, is necessary to evaluate the technical and economic feasibility of solar
energy to support the heating of greenhouses. It is essential to establish whether they can
produce an improvement in the profitability of farms.

3. Solar thermal energy to support the greenhouse heating
In addition to the purely agronomic reasons, agricultural production as any productive
activity, should pay special attention to the economic performance of the farm. The decision
to use heat in the greenhouse during the winter months depends on crop yields and the
evolution of crop prices and fuel. Among the most commonly used heating systems, the most
suitable for cultivation is done by convection-radiation with water as medium or high
temperature heat transfer fluid. However, the cost of such a facility is the highest within the
range of available systems. Therefore, we must not forget the usefulness of air heating
systems, less efficient but cheaper than the previous (Abril Hernández et al., 2007).
Solar thermal energy of low temperature for its use does not require complicated technology,
but represents a high cost of implementation for the most part caused by the collectors. This
limits the application of this technology in agriculture and farming. In order to evaluate the
technical and economic feasibility of implementing solar thermal energy to support the
heating of greenhouses, we analyze the production of pepper in substrate in El Campo de
Cartagena. It is an arid zone with warm winters (usually minimum temperatures above 0ºC in
December and January). The highest temperatures occur in July and August with values that
can exceed 40ºC. The mean values of humidity is maintained between 70 and 85%.
The pepper plant is in dire need of proper temperatures, so its control is of vital importance.
Optimum temperature varies with the stage of development although in general terms it is
estimated that during the day between 20 and 25ºC and at night from 16 to 18ºC. The
optimal value of the relative air humidity varies between 50 and 70%. It also requires high
light radiation throughout the growing season, especially in the early stages of development
and during flowering.
As representative of the study area are adopting a multi-tunnel greenhouse structure with
galvanized steel, three-layer upper enclosure through flexible polyethylene sheet and
Ethylene Vinyl Acetate 0.2 mm thick, and alveolar polycarbonate side closure of 6 mm thick.
The height of the channel is set at 4 m to the ridge at 6 m. The greenhouse is made by the
union of 5 modules of 8 m width each oriented in the east-west direction. Breeding lines are
north-south. A central corridor of 3 m of section facilitates cultivation. The length of the
modules is 50 m, ie, 2,000 m 2 of covered area, which may be representative of the growing
area. The greenhouse is equipped with a thermal shield allowing an energy saving of 30%.
To do a comparative study using three types of solar energy panels and other features, differ
in their energy performance and cost of acquisition. This is a collector made of plastic
materials and energy characterized at the University Miguel Hernández (Gomez, 2007) and
commercial collectors flat plate and evacuated tube. The energy parameters of the collectors
used in the calculations are shown in Table 1.
Table 1. Values of optical performance and the loss coefficient of the collectors used for the
installation of solar energy to support the greenhouse heating
Solar collector Optical performance Loss coefficient (W/m 2 ºC)
Plastic material 0.60 5.50
Flat plate 0.81 3.78
Evacuated-tube 0.84 1.75
Under these conditions, the heating power is around 180 W/m 2 and the annual energy
consumption is about 750 MJ/m 2 , distributed between October and May, spending 40% in
December and January. To prevent energy loss, the recommended solar contribution is
estimated at around 20% and it is achieved with a less than 20% of the greenhouse area.
The determination of the constituent elements of the installation of solar thermal energy as
support for greenhouse heating fuel is made from the guidelines and considerations made by

Peuser et al. (2005).
Economic evaluation involves considering the cost of the collector, amortized according to
life and operating costs of equipment and its comparison with conventional fuel.
The average cost of a flat plate solar collector is trading around 400 €/m 2 and vacuum tube
collectors on the 600 €/m 2 (IDAE, 2011). In these approximate values include the share of
support structure, anchor pads, manpower and installation materials, but does not consider
the costs incurred in preparing the ground location. The estimated half-life for both collectors
is 20 years. The plastic collector has an estimated cost of 130 €/m2 if deemed necessary
maintenance to ensure a lifetime of 20 years of all the components needed for installation,
including storage tank, which results economic study of the three collectors are comparable.
It is necessary to calculate the annual cost is the amortization of the installation of solar
collectors and compare the energy savings resulting from their use. However, the continued
upward trend of fuel prices makes economic estimates of the cost of heating greenhouses or
low energy consumption in this application of solar thermal energy have limited real validity.
Therefore, to perform the economic analysis of solar thermal power facility to support the
greenhouse heating adopting an approach which equals the unit cost of installing solar
energy with the cost savings derived from its use. This approach allows for the unit price of
fuel of choice from which solar thermal energy to support the greenhouse heating starts to be
profitable for the farmer. The calculation of this price is made for different energy demands
and its value is independent of fixed set point temperatures, so it is valid with any strategy of
the farmer. In all cases studied adopting a solar contribution of around 20%. Conventional
fuels considered are those used in El Campo de Cartagena, natural gas, gas oil, fuel oil and
propane. The expression used to calculate the price of fuel from occurring savings are
P = C A · A S · P FC / S · E H · A G (€/L)
where
P = Fuel prices. Its units depend on it (€/L, €/kg, €/kWh)
C A = Annual cost unitary or annual amortization of the installation of solar thermal (€/m 2 año)
A S = Area of solar thermal installation (m 2 )
P FC = Lower power fuel combustion (MJ/L; MJ/kg)
S = Solar contribution (dimensionless)
E H = Annual energy heating unit (MJ/m 2 año)
A G = Greenhouse area (m 2 )
The term does not include the boiler efficiency because its value varies greatly, as it can be
very low, especially in the case of conventional boilers with several years of service, or
exceed 100% from power lower combustion of fuel, if any condensation boilers with proper
functioning. Is necessary to consider those measures to improve energy efficiency in
generation of heat, such as increased combustion efficiency, use of fractional operation
burners, and heat recovery from exhaust fumes, the insulation of the heating system and
proper maintenance of heating systems. Therefore, the value obtained from the price of fuel
from which is produced by solar energy cost savings for support (Tables 2 to 4) are not exact
and precise amount for a given installation should be calculated from data obtained from a
reliable energy audit of the installation.

Tabla 2. Prices of fossil fuels from which it is profitable using plastic collectors with different
energy consumption
T
(ºC)
Consumption
(MJ/m 2 )
Collectors area
(m 2 )
Solar
contribution (%)
Natural gas
price (€/kWh)
Propane price
(€/kg)
Fuel price
(€/L)
Gas oil
price (€/L)
13 425 250 20.8
14 496 250 17.8
15 577 300 18.4
0.033 0.426 0.355 0.327
16 667 350 18.6
17 756 400 18.7
The installation of plastic collectors shown profitable in economic terms against the use of
natural gas when the unit price it exceeds 0.033 €/kWh. This value is independent of the
energy consumption of gases between 425 and 756 MJ/m 2 , ie does not depend on the
temperature setpoint taken between 13 and 17°C (Table 2). Such collectors are very
competitive as prices estimated for different fossil fuels are not too high.
When using flat plate collectors, there is solar energy savings when natural gas price
exceeds 0.047 €/kWh (Table 3). This is an increase greater than 40% over the use of plastic
collectors and occurs with the price of all fuels studied. Consequently, these sensors are not
very competitive for this application as the required fuel prices are high.
Tabla 3. Prices of fossil fuels from which it is profitable using flat plate collectors with different
energy consumption
T
(ºC)
Consumption
(MJ/m 2 )
Collectors area
(m 2 )
Solar
contribution (%)
Natural gas
price (€/kWh)
Propane price
(€/kg)
Fuel price
(€/L)
Gas oil
price (€/L)
13 425 100 18.1
14 496 120 18.6
15 577 150 20.0
0.047 0.603 0.502 0.463
16 667 160 18.4
17 756 180 18.3
The evacuated-tube collectors are competitive with natural gas when its price rises to 0.051
€/kWh (Table 4). This is an increase greater than 50% of the value of the price at which the
plastic collectors are competitive.
Tabla 4. Prices of fossil fuels from which it is profitable using evacuated-tube collectors with
different energy consumption
T
(ºC)
Consumption
(MJ/m 2 )
Collectors area
(m 2 )
Solar
contribution (%)
Natural gas
price (€/kWh)
Propane price
(€/kg)
Fuel price
(€/L)
Gas oil
price (€/L)
13 425 75 18.8
14 496 90 19.3
15 577 100 18.5
0.051 0.652 0.543 0.502
16 667 120 19.2
17 756 150 21.1
References
Abril Hernández, J.Mª., Cámara Zapata, J.Mª., Pascual, A., Madueño Luna, A. y Ruiz
Hernández, V. (2007). Evaluación de captadores de energía solar de bajo coste para
invernaderos. I Congresso Brasileiro de Energia Solar. ICBENS Libro de Resúmenes, pp. 45
Aramyan, L.H., Oude-Lansink, A.G.J.M. and Verstegen, J.A.A.M. 2007. Factors underlying
the investment decision in energy-saving systems in Dutch horticulture. Agricultural Systems,
vol. 94, pp. 520-527

Fernández-Zamudio, M.A., Pérez, A. y Caballero, P. 2006. Análisis económico de la
tecnología de los invernaderos mediterráneos: aplicación en la producción del pimiento.
ITEA, vol. 102, nº 3, pp. 260-277
García Martínez, M. C. 2009. La adopción de tecnología en los invernaderos hortícolas
mediterráneos. Tesis doctoral (340 pp). Universidad Politécnica de Valencia
Gómez Caballero, P. (2007). Alternativas constructivas en captadores de energía solar
térmica fabricados con materiales plásticos. Trabajo fin de carrera. Universidad Miguel
Hernández
IDAE (2011). Plan de Energías Renovables, 2011-2020
MAPA (2012). Anuario de estadística agroalimentaria
Montero, J.I., Stanghellini, C. y Castilla, N. 2008. Invernadero para la producción sostenible
en áreas de clima de invierno suaves. Horticultura Internacional, nº 65, pp. 12-26
Peuser F. A., Remmers K. y Schnauss M. (2005). Sistemas solares térmicos. Diseño e
instalación. PROGENSA. Solarpraxis AG. ISBN: 84-95693-20-8. 392 pp