DESIGN, REALIZATION, TESTS, COMPARATIVE ANALYSIS OF LOW ELECTRIC CONSUMPTION PHOTOVOLTAIC COOLING SYSTEMS
COSTIC - E.MICHEL - F.BONNEFOI -Rue A Lavoisier - Z.I Saint-Christophe - 04000 DIGNE - FRANCE
Phone: 33.4.92.31.19.30 - Fax: 33.4.92.32.45.71 - Email:
CIEMAT - F.ROSILLO - 22 avenida complutense - 28040 MADRID - SPAIN
Phone: 34.1.346.6675 - Fax: 34.1.346.6037 - Email:
ALPES FROID - P.RICHARD - avenue de la gare - 04700 ORAISON - FRANCE
Phone: 33.4.92.78.62.09 - Fax: 33.4.92.78.68.74
APEX - A.MINE - E. LAGET - 4 rue de l’industrie - 34880 LAVERUNE - FRANCE
Phone: 33.4.67.07.02.02 - Fax: 33.4.67.69.17.34 - Email:
UNIVERSITY OF ATHENS - A.ARGIRIOU - P.O BOX 20048 - GR-118 10 ATHENS - GREECE
Phone: 30.1..342.1270 - Fax: 30.1.346.4412 - Email:
ABSTRACT:
The aim of this project funded by the European Commission is the following one: determine low electric consumption cooling systems well adapted for photovoltaic and for application in developing countries.
Two different systems were intended: evaporative air-cooler, buried pipe system.
The efficiencies in sensible heat, quite high for these systems (6 to 10) in application with 220VAC, have been optimised for photovoltaic. The tests showed that these coefficients could exceed values of 20.
The innovative aspect of the studied systems are:
low consumption motor for fans
adapted fan for different systems
range of device defined in order to provide cooling power necessary for premises from an office to a little shop
cooling "along with the sun"
Keywords: Ventilation, Stand-alone PV Systems, Developing countries
1.Presentation of the cooling systems
1.1.Evaporative air cooler
- Principle:
The hot air is humidified in contact with water so that to get cool by losing as much sensible heat as latent heat of vaporisation it gives to water.
The contact air-water may be made by a humid section, or by atomisers.
The air evolution can be represented on a psychrometric diagram : it is characterised by a constant wet bulb temperature. Thus, the air goes out of the air cooler with a dry bulb temperature lower and a relative humidity higher.
The efficiency of those air coolers is :
e :dry bulb temperature of the air at the inlet
s :dry bulb temperature of the air at the outlet
h :wet bulb temperature of the air
The sensible frigorific capacity in [W] is :
Pf = 0.34 Q (e - s)
with Q : volumetric air flow rate in [m3.h-1]
The coefficient of performance is : COP = Pf / Pel
with Pel : electric power absorbed in [W]
We have to distinguish two types of evaporative systems : direct and indirect.
- Direct evaporative air-cooler
In those devices, the air to blow in premises is directly in contact with water.
The exchange air-water can be made by several ways :
Plane Pad
The evaporative air cooler contains humidification plates (in wood or PVC) on which water, brought by a pump, flows.
A fan (in general, a centrifugal one) pumps in the air through plates, and blows it in the premises. The efficiency can reach 80%.
Rotary Pad
The pad has a drum shape : it is humidified and washed, turning in the water tank. The air goes through the drum thickness and is cooled. Thus, the water pump is removed.
Pulverisation
The air goes through water spray towards an evaporative and filtering plate and after towards another plate which traps the humidity obtained. The efficiency can reach 80%.
This type of direct evaporative air cooler is well-adapted for dry and hot climates. However, we must be careful with stagnant water, because of risks of legionellosa.
- Indirect evaporative air-cooler
Principle
The air is cooled by direct evaporation. Then, it cools new air through an exchanger air-air.
Thus, we obtain cool air, without supplies of humidity. Moreover, the risks of legionellosa are limited, since the air in contact with water is thrown back outside.
In this case, the efficiency is :
With :
t1e : dry bulb temperature of the primary air at the inlet
t1s: dry bulb temperature of the primary air at the outlet
2e : wet bulb temperature of the secondary air at the inlet
We get efficiencies between 40 and 80%.
The use of that kind of air cooler can be broadened to countries less hot and less dry, for cooling only, or in complement with a classic air conditioning system, to reduce the load of the refrigerating compressor and thus, to reduce the costs of exploitation.
1.2.Cooling with buried pipes
During hot period the temperature of the ground can be lower that the temperature of the air.
This is the case for the Mediterranean climate. The same principle is used in the USA and Canada, to warm up the air in winter.
The exterior air is blown in a buried pipe and is cooled by forced convection and then it is blown in the premises.
The ground is an interesting source of freshness because its temperature decreases while the depth increases.
Figure 1 - Evolution of temperature for different depths into the ground (Athens - GREECE)
Furthermore, the temperature of the ground less and less depends on the daily variations of the exterior and the peaks of temperature don't take place at the same moment.
However, the performances of such a system can decrease during an extended use, on account of the heat accumulation in the ground.
So as to avoid this problem, we can improve the heat transfer by laying sand around the pipes.
We also can let the installation running all night long, if the exterior temperature is low enough to disperse the heat stored during the day.
Studies brought the following conclusions :
-The more the pipe is long, the more the temperature at the outlet decreases (beyond a certain value, the length doesn't bring improvements anymore).
-The more the radius of the pipe is long, the more the temperature at the outlet increases (because the coefficient of the convective transfer is lower).
-The more the air speed is big, the more the temperature at the outlet increases.
-The more the depth is big, the more the temperature at the outlet decreases (beyond 6.30m, the decrease in temperature isn't significant).
Naturally, those parameters depend on the nature of the ground, particularly on its humidity and on its diffusivity.
On the other hand, the choice of the pipe diameter must take into account the losses of head it generates and thus its influence upon the fan sizing.
2.PV systems
All partners have contributed to develop two different direct evaporative air cooler prototypes with rotary pads and vertical pads and one buried pipes prototype supplied with photovoltaic (PV) energy. These prototypes have been studied to run along the sun thanks to an impedance adapter. The photovoltaic system sized by the partner APEX in collaboration with COSTIC, based on the design information provided by the University of Athens, coupled with the fans specifically designed for the needs of the current project by the partner ALPES FROID, showed a reliable and robust behaviour throughout the experimental period. No failures were observed and the system operated without any interruptions.
2.1.Conception of electronic command to manage the systems along the sun
In the field of photovoltaic solar energy, the term optimizer or impedance adapter is applied to electronic devices which essential function is to allow an optimal transfer of energy between a generator and a receptor (between the solar modules and a DC motor). From the electronic point of view, these are DC/DC voltage converters which often lower voltage using the MPPT ( Maximum Power Point Tracker) function. On a solar module, the maximum power point is located on a specific point of the U/I Curve and any move away from this point brings a loss of power. The optimiser prevents this phenomenon.
The main advantage is a better energy transfer between the solar generator and the receptor. The figures 2 and 3 show the influence of impedance adapter on functioning of fans for the same solar irradiation.
This system presents different advantages:
- Optimum sizing of the collectors surface (number of modules) to the energy consumption level
- The optimizer increases the yield of the photovoltaïc output, (up to 18% in summer and up to 30% in winter)
- The possible collector’s discount may be several times the price of the P.C.E - Optimizer
- The optimizer allows a direct supply -ie electric motor-
Figure 2: Incidence of impedance adapter
Figure 3: Fan functioning with and without I.A. for the same conditions of radiation
Figures 4 and 5 indicate schemes of the cooling PV systems : buried pipes and evaporative air cooler
Figure 4:PV modules coupled with the fan of the buried pipes thanks to an impedance adapter
Figure 5: PV modules coupled with the fan of the evaporative air cooler thanks to an impedance adapter
The only electric consumption for the two cooling systems are for fans and pump (for evaporative device). In both case the performance coefficient (COP) is determined as Pf / Pe (with Pf : frigorific capacity and Pe : electric power absorbed).
2.2. Tests results with buried pipes
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Figure 6 - Optimal data for a buried pipes installation
Optimal data to design buried pipes installation are indicated on the figure 6.
Main results obtained by tests in laboratory are summarized in table 1. It shows that for a given temperature of ground the outlet temperature is independant of inlet one for a pipe length of minimum 30 m.. At this distance air temperature in pipe is approximatively the ground temperature: 20 °C of air temperature for 18 °C of ground temperature (fig. 7)
We obtain a cooling power of 1,7kW for a pipe length of 30 to 40 m, buried at 1,5m, with a diameter of 200 mm, an air speed of 3 m/s, an outside temperature of 35°C and a ground temperature of 18°C..
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Inlet temperature [°C] / Outlet temperature [°C] / Ground temperature [°C] / COP30 / 20 / 18 / 14,7
35 / 20 / 18 / 20,8
40 / 20 / 18 / 27
Table 1 Performance for an installation of 3 pipes with air flow 1 100 [m3/h] and fan power 245 [W]
2.3. Tests results with direct evaporative air cooler
Performances (in dry climate)
The evaporative air cooler designed by COSTIC and ALPES FROID can cool the air until 80% of the gap between the dry bulb temperature and the wet bulb temperature.
The average performance (COP) for a sunny day can reach 40.
Water consumption concerning a building of 225 m3, i.e. about 90 m2 cooled from 15th June to 15th September = 2 to 3 m3 of water ; daily consumption : 0,020 to 0,035 m3.
Results of performance with evaporative air cooler (Max flow rate : 1500m³/h ; Toutside=40°C ; Hr=25%) are indicated in fig. 8.
Figure 8: Performance of evaporative air cooler according to air flow
3.Tests in real conditions
3.1.Test results of the evaporative air cooler
The evaporative air coolers designed by COSTIC and ALPES FROID can cool the air until 80% of the gap between the dry bulb temperature and the wet bulb temperature.
Prototype has been tested in CIEMAT at Madrid (photos 1 and 2).The average performances obtained (COP) for a sunny day can reach 40. The average COP of the system is about 30 for a working during summer in Madrid.
The water consumption concerning a building of 225 m3, i.e. about 90 m2 cooled from 15th June to 15th September is 2 to 3 m3 of water ; daily consumption : 0,020 to 0,035 m3.
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3.2.Test results of the buried pipes
Buried pipes has been tested during summer at National OBSERVATORY OF Athens (photos 3 and 4)
The average COP of the system in real conditions is about 12 (majority between 5 to 25) for a length of tube equal to 20m. For the optimum length 30m, we can improve the COP to 18.
Photos 3 and 4- PV buried pipes prototype in Athens
The dampers designed in order to maintain the air flow across the exchanger close to 3 m.s-1, operated as expected since the major part of the velocity values recorded were between 2.5 and 3.5 m.s-1.
The obtained temperature difference across the exchanger is satisfactory.
The ground, due to its important thermal inertia, can provide cooling throughout the whole summer. This is because the ground temperature takes its highest value at the end of the cooling season, when the cooling requirements become lower.
The performance of the system is not affected for distances between exchanger tubes greater than 0,5 - 1 m. Closer distances were not considered, since it is not likely to encounter them in practice.
4.Economical and technical aspects
Several simulations on TRNSYS allow to define the performance of the prototypes and its influence on the thermal behaviour of different typical buildings (office and dwelling). Analysis of simulation and experimental results allow to draw some conclusions about the performances, economical aspects.
4.1.PV evaporative air cooler
For building with medium heat loads located in hot and dry climate, the PV evaporative air cooler can reduce the indoor temperature by 3 to 5 K in comparison with the same building without cooling.
The air temperature of the indoor space should be around 2 K higher than the discharge air temperature and its relative humidity should be below 70%.
The resulting temperature of the indoor space is 4 to 7 K below the outdoor dry bulb temperature.
Concerning the costs, the PV evaporative air cooler is more competitive in term of cost in comparison with classic systems. The cost of functioning is very low in comparison with a conventional system. But we have to add the water consumption, 50 l/day (case for a dwelling of 100 – 150m2). This consumption can be penalising in certain regions where water is rare.
PV evaporative air cooler offers an interesting alternative for cooling. It can provide cooling, just consuming the necessary electricity for the operation of air fans and for a small water pump. This is much less compared to the electricity required for the operation of conventional air-conditioners.
In case of building with high heat loads, this system can be coupled with other systems as :
-Night cooling
-System of air distribution
-Traditional system with mechanical compression (so that to limit the installed power).
4.2.PV buried pipes
The investment cost of the PV buried pipes installation is a little higher than the investment cost of a classic air conditioning system (fig. 9)
Figure 9: Comparison between the total investment costs (Tax free) of PV buried pipes and classic air conditioners (multi-split) for different dwelling surfaces.
Energetic costs: very low with alternative current, free with photovoltaic supply.
Maintenance costs: very low. There are no major operating and maintenance costs associated with buried pipes except for an occasional cleaning of the system. The life span of the system is about equal to the buildings ones themselves.
In case of very hot climate, the main advantage of this system is that it reduces peak demand for cooling and heating. This not only reduces energy costs, but also overall equipment and installation costs, since smaller dimensions may be used.
Economical : low electrical consumption with photovoltaic supply or AC supply.
Environmental friendly : it doesn’t use freon fluids, nor compressor.
Performance : coefficients of performance can reach 20.
Maintenance is limited.
Within the context of a photovoltaic solution, the system run along the sun.
PV buried pipes system is attractive for cooling and preheating ventilation air for many buildings in mild climatic regions. The resulting temperature of the indoor space should be about 3 to 5 K below resulting temperature of the indoor space without cooling.
The technology is already in successful use in several commercial, school and residential buildings. Maybe, we can hope to see the development for smallest buildings : dwellings, little shops, little offices…
5.Conclusions
These products can be used as an air conditioning systems in developing countries and Mediterranean countries. With these systems, it could be possible to cool premises such as little office buildings or dwellings. It is not possible to ensure the conditions of comfort all along the hot season as these systems only allow to reduce the temperature inside the premises of 3 to 4 °C compared to the situation without any conventional air conditioning systems. For instance, in the south of France it is possible to reach these conditions during about 80 to 90 % of summer.
At the moment these premises can be treated by an air conditioning split system with a heat pump as cold generator. A financial study, undertaken during the study, showed that costs of conventional systems compared to the costs of the new devices developed during the programme are rather equal. But these new products present the benefits to use a renewable energy which means that the working costs are low.
These systems present a disadvantage when the heat loads are important : it is not possible to ensure conditions of comfort. So there is an important work to do so that explain, to potential customers, the limit of the systems in order to avoid any bad references. Thus, it would be interesting to obtain , through a demonstrating programme, some examples of successful installations to convince potential users.
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E.MICHEL - F.ROUESNE: Rapport d’étude d’un climatiseur évaporatif adapté au climat sahelien, 1989.
ASHRAE Handbook, HVAC systems and equipments, Evaporative air cooling, 1992.
P.BLONDEAU, M.SPERANDO, L.SANDU:
Potentialités de la ventilation nocturne pour le rafraîchissement des bâtiments du sud de l’Europe. Conférence européenne sur la performance énergétique et la qualité des ambiances dans le bâtiment, November 94.
M.SANTANOURIS, D.ASIMAKOPOULOS:
Natural Cooling Techniques, May 1995.
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