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2009 to 2014, the joint research program ZINEG is carried out in Germany. Its main aim is to reduce the consumption of fossil fuels and hence the CO2 emissions up to 90% for production in greenhouses. At Humboldt University the research is focused on using a greenhouse system as solar thermal collector with above-ground heat storage. During the tomato production in 2011, a seasonal energy efficiency ratio (SEER) of 5.1 and a heating seasonal performance factor (HSPF) of 4.4 was achieved using an electrically-driven heat pump utilized for cooling and heating in the collector greenhouse. Approximately half of the solar irradiation was stored into an insulated rainwater-tank. This corresponds to 1.76 GJ/m2. Twenty percent of this solar energy was collected by condensation (originally latent heat) on finned pipes in the roof zone. For heating the collector greenhouse, about 0.53 GJ/m2 of the stored heat was re-used. That means that additional heat might be exported or the cooling surface area can be reduced to one-third. Furthermore, the solar thermal collector greenhouse achieved a primary energy consumption of 147.6 MJ/m2 (considering a full re-use of the stored heat). Simultaneously, the conventional greenhouse achieved a primary energy consumption of 767.1 MJ/m2. That means that the consumption of non-renewable energies (fossil fuels) is reduced up to 81% with the collector system. In further studies an economic assessment regarding energy-saving in semi-closed greenhouses should be performed to estimate the potential of such facilities. http://www.actahort.org/books/1037/1037_20.htm
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ZINEG Project - Energetic Evaluation of a Solar Collector Greenhouse
with Above-Ground Heat Storage in Germany
I. Schucha, D. Dannehl, L. Miranda-Trujillo, T. Rocksch and U. Schmidt
Biosystems Engineering Division, Humboldt-Universität zu Berlin
Albrecht-Thaer-Weg 3, D-14195 Berlin
Germany
Keywords: semi-closed, solar energy, heat pump, cooling, COP, collector efficiency
Abstract
2009 to 2014, the joint research program ZINEG is carried out in Germany.
Its main aim is to reduce the consumption of fossil fuels and hence the CO2 emissions
up to 90% for production in greenhouses. At Humboldt University the research is
focused on using a greenhouse system as solar thermal collector with above-ground
heat storage. During the tomato production in 2011, a seasonal energy efficiency
ratio (SEER) of 5.1 and a heating seasonal performance factor (HSPF) of 4.4 was
achieved using an electrically-driven heat pump utilized for cooling and heating in
the collector greenhouse. Approximately half of the solar irradiation was stored into
an insulated rainwater-tank. This corresponds to 1.76 GJ/m2. Twenty percent of this
solar energy was collected by condensation (originally latent heat) on finned pipes in
the roof zone. For heating the collector greenhouse, about 0.53 GJ/m2 of the stored
heat was re-used. That means that additional heat might be exported or the cooling
surface area can be reduced to one-third. Furthermore, the solar thermal collector
greenhouse achieved a primary energy consumption of 147.6 MJ/m² (considering a
full re-use of the stored heat). Simultaneously, the conventional greenhouse achieved
a primary energy consumption of 767.1 MJ/m². That means that the consumption of
non-renewable energies (fossil fuels) is reduced up to 81% with the collector system.
In further studies an economic assessment regarding energy-saving in semi-closed
greenhouses should be performed to estimate the potential of such facilities.
INTRODUCTION
Due to the transmittance of solar radiation and the warming of the indoor air,
greenhouses are generally comparable to solar thermal collectors. Thus, a bivalent use
seems to be possible.
Already in the 1980’s technical solutions were developed, in which excess heat
was removed by forced-air heat exchangers in greenhouses (Damrath, 1982). Since the
late 1990’s the development of concepts for energy saving and sustainable energy supply
was pushed intensively. Main priorities were and are the reduction of heat consumption
by new glazing materials and the optimization of climate control strategies, but also the
reduction of fossil fuels by solar heating with seasonal heat storage and the use of heat
pumps. In 2004, the first commercial closed greenhouse was built (1.4 ha) including heat
pump, refrigeration, aquifer heat storage and low temperature heating. In this context, a
higher yield of 22%, a water saving of 50% and an energy saving potential up to 33%
were achieved (de Gelder et al., 2005; Opdam et al., 2005). From an energetic point of

a mail@ingo-schuch.eu
view, it was found that a closed greenhouse area of 25% should be combined with an
open one of 75% (Heuvelink et al., 2008). Alternatively, a closed operation mode in
greenhouses may be preferably applied in autumn, winter and spring, whereas a semi-
closed system is more effective in summer (Yildiz and Stombaugh, 2006). Concerning the
use of heat pumps in greenhouses, Grisey et al. (2012) found a mean coefficient of
performance (COP) of 3.7 when using a water-water heat pump for aquifer thermal
energy storage. Kougias et al. (2012) calculated a COP of 4.25 for a geothermal heat
pump. Migeon et al. (2012) used an electrical heat pump, which achieved a COP of 6.4
(considering only compressor power) and 4.6 (considering total power).
From 2009-2014, the joint research program ZINEG (the low energy greenhouse)
is carried out in Germany. One important aim is to reduce the consumption of fossil fuels
up to 90% for production in greenhouses (Tantau et al., 2011). Thus, at Humboldt
University a new greenhouse system used as solar thermal collector with above-ground
heat storage was developed. Related to this, a study about the possibilities of uncoupling,
storage and re-use of solar heat and determination of saving potentials is carried out. First
results of the start-up phase about the evaluation of energy harvesting under summer
conditions were already presented by Schmidt et al. (2012).
MATERIALS AND METHODS
Experimental Setup
The experiment was carried out in two N-S oriented Venlo-type glasshouses
named solar collector and conventional greenhouse. These are located in eastern Germany
(52° 28 2.28′′ N, 13° 17 57.88′′ E). Each of the greenhouses had a ground area of
307 m2, 6 m standing walls and an overall height of 6.7 m. They were covered with 4 mm
single glass panes on the roof and 4, 8, 4 mm double glass panes with argon gas inside on
side walls. In addition, a conventional hot-water pipe rail heating system (about 90 °C),
five tubular-film heat blowers (< 50 °C) and a vegetation heating system (< 40 °C) were
installed in each greenhouse. In order to save energy at night, a thermal screen (XLS 10
ULTRA REVOLUX, Svensson, Sweden) was installed in both greenhouses. However,
only the collector greenhouse was equipped with aluminized energy screens in the roof
zone (XLS 18 REVOLUX, Svensson, Sweden) and the standing wall regions (ILS 70
ULTRA, Svensson, Sweden). Nearly 500 tomato plants (cv. Pannovy) per greenhouse
were cultivated in a gully hydroponic growing system with drip irrigation, fog system and
CO2 enrichment (800 ppm at closed vents).
The greenhouses were built to compare the semi-closed operation mode (vents at
29 °C) with the conventional one (vents at 24 °C). Unlike the conventional greenhouse,
the semi-closed system was used as solar thermal collector with electrically-driven water-
water heat pump (128 W/m2) for greenhouse cooling and heating (Fig. 1). In this context,
finned pipes with a total length of 342 m (corresponds to 1.1 m/m2), as well as condensate
channels in the roof zone were used for cooling and dehumidification caused by natural
convection and condensation. The ratio of the cooling surface to the air volume was
0.35 m2/m3 (corresponds to 2.2 m2/m2). The maximum cooling capacity of the finned
pipes was 60 W/m3 (corresponds to 380 W/m2). For heat-storage (range 7-42 °C, max
138 MJ/m³), an insulated (polystyrene, total thickness of 40 mm) rainwater-tank with
0.85 m3/m² was used. The stored solar heat was re-used for low-temperature heating using
tubular-film heat blowers as well as vegetation heaters.
Evaluation Methods
The collector efficiency  is an important value for evaluating the solar thermal
collector greenhouse. It describes the fraction of solar irradiation , [ J ] on the overall
horizontal greenhouse surface area which is converted into stored heat , [ J ].
 ,
,
(1)
Taking into account the electrical work for heat pump cooling and required
circulation pumps , [ J ], the calculation of net collector efficiency , can be
done.
, ,
, 
, (2)
In the collector greenhouse, a heat pump was used to combine cooling and heating
operation modes. The parameter that evaluates the current situation of this thermal
process is described by the coefficient of performance (COP). The seasonal energy
efficiency ratio (SEER) for cooling mode can be derived from COP measurements over a
certain period. The heating seasonal performance factor (HSPF) results from the heating
mode over a certain period. Related to this,  [ J ] indicates the amount of cooling
energy and  [ J ] the amount of heating energy that was generated by electrical
work of the heat pump compressor for cooling  [ J ] or heating
 [ J ].
  
 (3)
  
 (4)
Taking into account the electrical work for thermal and circulation processes
 [ J ], the calculation of  and  can be done.
 

,
 (5)
 

,
 (6)
The solar thermal collector greenhouse used an electrically-driven heat pump and
a couple of electrically-driven circulation pumps. Therefore, a primary energy factor for
Germany of 2.6 (EnEV, 2012) was multiplied in equation (3) to (6) regarding to electrical
work. The resulting parameter is called primary energy ratio (PER) and indicates the
substitution of fossil fuels and CO2 emissions (PER > 1) by using renewable energy
technologies (e.g. heat pumps).
RESULTS AND DISCUSSION
Energetic parameters of the 6-month start-up phase in 2010 and the 8-month
tomato season of the following year in the collector greenhouse were compared in table 1.
In 2011 season, a collector efficiency of 0.48 (mean) was achieved. Taking into account
the overall electrical work for heat pump cooling and circulation processes, a mean net
collector efficiency of 0.42 was reached by the system. Thus, about half of the irradiated
solar heat was stored (1755 MJ/m2) into the insulated rainwater-tank (Fig. 2). Twenty
percent (mean) of this seasonal energy input was caused by condensation (144 l/m²) on
the finned pipe heat exchangers in the roof zone of the greenhouse. In 2011 season, the
collector efficiency was slightly lower than that in the previous year, because the lower
collector efficiency in springtime wasn’t included in 2010 season.
For heat storage, an insulated (polystyrene, thickness of 40 mm) above-ground
rainwater tank was used. Based on an additional insulation of the rainwater tank, only 8%
of the stored heat was heat loss in 2011 season, whereas in previous year without
insulation a heat loss of 34% was determined by Schmidt et al. (2012).
To heat the collector greenhouse, about 534 MJ/m2 of the stored heat was re-used
(see Fig. 2). This means that additional heat might be exported to further greenhouses.
Alternatively, the cooling surface area can be reduced to one-third. In this context, heat
was removed from the above-ground heat storage to cover the heating base load of the
conventional greenhouse (469 MJ/m²). Additionally, a cooling tower was used.
Furthermore, the seasonal energy efficiency Ratio (SEER) and heating seasonal
performance factor (HSPF) of 2010 and 2011 season were compared in table 1. In 2011
season, a SEER of 5.1 and HSPF of 4.4 were achieved using an electrically-driven water-
water heat pump (electricity 128 W/m2), which was used for cooling/heating processes in
the collector greenhouse. These values are higher than those reported by Grisey et al.
(2012) and Kougias et al. (2012). However, Migeon et al. (2012) achieved a higher COP
using an electrically-driven air-water heat pump. Taking into account the electrical work
for thermal as well as circulation processes, a SEERtotal of 3.3 and HSPFtotal of 2.9 were
reached. These values are similar to the results of the start-up phase in 2010 season,
which were published by Schmidt et al. (2012).
Furthermore, the solar thermal collector greenhouse achieved a primary energy
consumption of 147.6 MJ/m² (considering full re-use of stored heat). Simultaneously, the
conventional greenhouse achieved a primary energy consumption of 767.1 MJ/m². That
means that the consumption of non-renewable energies (fossil fuels) is reduced up to 81%
with the collector system.
CONCLUSIONS
The use of solar thermal energy has the great potential to reduce the consumption
of fossil fuels. The ZINEG solar collector greenhouse can be considered as a useful tool
to save high amounts of fossil fuels up to 81%. Thus, about half of the solar irradiation
was stored into the well insulated above-ground heat storage. This means, additional heat
might be exported to further greenhouses. Alternatively, the cooling surface area can be
reduced to one-third. Furthermore, a SEER of 5.1 and HSPF of 4.4 were achieved using a
heat pump for cooling/heating processes in the solar collector greenhouse.
In further studies the relation of energy-saving potential and economic feasibility
should be tested.
Acknowledgements
Thanks to the German Federal Ministry for the Environment, Nature Conservation
and Nuclear Safety (BMU) and the Rentenbank managed by the Federal Ministry of
Food, Agriculture and Consumer Protection (BMELV) with assistance of the Federal
Office for Agriculture and Food (BLE).
Literature Cited
Damrath, J. 1982. Solarenergienutzung im Gewächshaus. Eine energetische Darstellung
des doppelbedachten Gewächshauses mit solarunterstützter Heizung (Solar energy in
greenhouses. An energetic description of the double roofed greenhouse with solar
assisted heating). Gartenbautechnische Informationen, Heft 14, Hannover.
de Gelder, A., Heuvelink, E. and Opdam, J.J.G. 2005. Tomato yield in a closed
greenhouse and comparision with simulated yields in closed and conventional
greenhouses. Acta Hort. 691:549-552.
EnEV 2012. Energieeinsparverordnung (German Energy Saving Ordinance). Federal
Ministry of Justice, Berlin.
Grisey, A., Brajeul, E., Tisiot, R., Rosso, L., D'amaral, F. und Schueller, M. 2012. Energy
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in aquifer. Acta Hort. 952:509-514.
Heuvelink, E., Bakker, M., Marcelis, L.F.M. und Raaphorst, M. 2008. Climate and yield
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Tables
Tab. 1. Energetic parameters of cooling and heating in the semi-closed solar collector
greenhouse in 2010 and 2011 season with tomato cultivation.
Parameter1 Mean 20102Mean 20113
Collector efficiency 0.55 0.48
Collector efficiencyne
t
0.47 0.42
HSPF 4.6 4.4
HSPFtotal 3.3 2.9
SEER 5.0 5.1
SEERtotal 3.6 3.3
PER heating 1.8 1.7
PER heatingtotal 1.3 1.1
PER cooling 1.9 1.9
PER coolingtotal 1.4 1.3
1HSPF/SEER calculated with values >1 and <8; PER calculated with primary energy factor 2.6 (electricity).
2Given period 6.5.2010 - 16.11.2010.
3Given period 8.3.2011 - 22.11.2011.
Figures
Fig. 1. Technical concept of the semi-closed solar collector greenhouse with heat pump
cooling/heating and above-ground heat storage.
Fig. 2. Energy flows of the solar collector system in 2011 season (8.3.-22.11.2011) with
tomato cultivation in eastern Germany.
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Closed and semiclosed greenhouses are used to collect thermal solar energy and for cooling technology. Other than the problem of long-term energy storage in closed greenhouses, the assembly and capacity of cooling and heating systems and the operational control of the microclimate and energy management system are the focus of research and development. A new prototype of solar greenhouse was constructed at Humboldt University, Berlin. In a 300 m2 Venlo-type greenhouse, a cooling fin system under the roof was connected to a heat pump and a low temperature storage tank. Several operational modes are possible for heating and/or cooling the greenhouse and to charge or discharge thermal storage using new technology for integration of the heat pump into the hydraulic pipe system. First results of perational behavior are shown by using typical evaluation parameters such as cooling capacity, collector efficiency, and the coefficient of performance for the heating and cooling operation. With the cooling fin ystem (cooling surface 1900 m2), the cooling capacity increased to 400 W m-2. The latent to sensible heat exchange ratio was about 40%. The maximum amount of water that condensed on the fin surface as 1.2 L (m2 day)-1. During the experimental interval from May to October 2010, the collector efficiency of the solar greenhouse was 0.55-0.8. The heat pump worked for greenhouse cooling with a seasonal energy efficiency ratio (SEER) of 4.9 and for heating with a heating season performance factor (HSPF) of 4.6.
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The so-called closed greenhouse (closed ventilation windows) is a recent innovation in Dutch greenhouse industry. The technical concept consists of a heat pump, underground (aquifer) seasonal energy storage as well as daytime storage, air treatment units with heat exchangers, and air distribution ducts. Savings of up to 30% in fossil fuel and production increases by up to 20%, mainly because of the continuously high CO2 concentration, have been reported. Economic feasibility of this innovative greenhouse highly depends on the yield increase that can be obtained. In this simulation study the effects of greenhouse climate on tomato yield in a closed greenhouse are presented. The explanatory model INTKAM was used, which has several submodels e.g. for light interception, leaf photosynthesis and biomass partitioning. The closed greenhouse offers possibilities for combinations of light, temperature, air humidity and CO2 concentration that are impossible in a conventional greenhouse. At high CO2 concentration and high light intensity, leaf photosynthesis shows a more narrow optimum for temperature than at high CO2 and moderate light intensity. However, the response of crop photosynthesis to temperature has a much broader optimum than that of leaf photosynthesis. Besides photosynthesis, temperature also influences aspects like partitioning, leaf area development and fruit development. Yield potential reduces at temperatures above 26°C, with fruit set being one of the first processes that is negatively influenced by supra-optimal temperatures. Based on actual climatic conditions in a conventional and a closed greenhouse (same crop management) measured during two years, INTKAM predicts an increase in yield by about 17%. Hence, in a closed greenhouse a higher stem density can be maintained for obtaining the same average fruit weight (size) as in a conventional greenhouse. In 2005 actual yield increase was similar to the simulated one (16%), but in 2004 only a 9% higher yield was realized, at least partly because of botrytis infection in the closed greenhouse.
Eine energetische Darstellung des doppelbedachten Gewächshauses mit solarunterstützter Heizung (Solar energy in greenhouses. An energetic description of the double roofed greenhouse with solar assisted heating). Gartenbautechnische Informationen, Heft 14
  • J Damrath
  • A De Gelder
  • E Heuvelink
  • J J G Opdam
Damrath, J. 1982. Solarenergienutzung im Gewächshaus. Eine energetische Darstellung des doppelbedachten Gewächshauses mit solarunterstützter Heizung (Solar energy in greenhouses. An energetic description of the double roofed greenhouse with solar assisted heating). Gartenbautechnische Informationen, Heft 14, Hannover. de Gelder, A., Heuvelink, E. and Opdam, J.J.G. 2005. Tomato yield in a closed greenhouse and comparision with simulated yields in closed and conventional greenhouses. Acta Hort. 691:549-552.