Cooling with Solar Heat: Growing Interest in Solar
Air Conditioning
Sunny summer days are beautiful, yet in the office a hot day can
be altogether stressful. Because productivity can suffer under such
conditions, more and more buildings are being fitted with air-conditioning
systems. This is where solar air conditioning comes in: The summer
sun, which heats up offices, also delivers the energy to cool them.
The thermal use of solar energy offers itself: Days that have the
greatest need for cooling are also the very same days that offer
the maximum possible solar energy gain.
The demand for air conditioning in offices, hotels, laboratories
or public buildings such as museums is considerable. This is true
not only in southern Europe, but also in Germany and middle Europe.
Under adequate conditions, solar and solar-assisted air conditioning
systems can be reasonable alternatives to conventional air conditioning
systems. Such systems have advantages over those that use problematic
coolants (CFCs), not to mention the incidental CO2 emissions that
are taking on increasingly critical values.
Sorption-assisted air conditioning: collector system
on the rooftop of Chamber of Commerce and Industry in Freiburg,
Germany. Photo: Fraunhofer ISE.
The trend towards solar-assisted air conditioning is met by the
organizers of the forum "Solar assisted Air-Conditioning of Buildings"
at the convention Intersolar 2002: The German Association for Solar
Energy (Die Deutsche Gesellschaft für Sonnenenergie (DGS)), the
Fraunhofer Institute for Solar Energy Systems (Fraunhofer Institut
für Solare Energiesysteme ISE), the Institute for Maintenance
and Modernization of Buildings at the Technical University of Berlin
(Institut für Erhaltung und Modernisierung von Bauwerken e.V.
an der TU Berlin), and the Pforzheimer Solar Promotion Corporation
(Pforzheimer Solarpromotion GmbH) are all offering a two-day international
forum on the state of technology, the energy and economic aspects
of solar cooling as well as the possible fields of application. Next
to German companies, organizations from the entire world have registered
including firms from Israel, Ghana, Spain, India, the Netherlands,
Belgium, and Austria. This Solar-Report will briefly inform you over
the possibilities and technology of solar air conditioning and will
also cover economic aspects.
Basic structure of a solar air conditioning
system
What is Solar Air Conditioning?
Should buildings be cooled with the help of solar energy, then
water-assisted air conditioning systems or ventilation systems can
be powered with heat that is made available by solar collectors.
No long-term intermediate storage is necessary in months of high
solar energy gain or in southern lands. The sun can, at least seasonally
at our latitudes, provide a substantial part of the energy needed
for air conditioning. Combination water-assisted systems and ventilation
systems are also possibilities.
How does Solar Air conditioning Work?
The basic principle behind (solar-) thermal driven cooling is the
thermo-chemical process of sorption: a liquid or gaseous substance
is either attached to a solid, porous material (adsorption) or is
taken in by a liquid or solid material (absorption).
The sorbent (i.e. silica gel, a substance with a large inner surface
area) is provided with heat (i.e. from a solar heater) and is dehumidified.
After this "drying", or desorption, the process can be
repeated in the opposite direction. When providing water vapor or
steam, it is stored in the porous storage medium (adsorption) and
simultaneously heat is released.
Processes are differentiated between closed refrigerant circulation
systems (for producing cold water) and open systems according to
the way in which the process is carried out: that is, whether or
not the refrigerant comes into contact with the atmosphere. The
latter is used for dehumidification and evaporative cooling. Both
processes can further be classified according to either liquid or
solid sorbents. In addition to the available refrigerating capacity,
the relationship between drive heat and realized cold energy (coefficient
of performance; COP) is also an essential performance figure of
such systems (see Table 1 at end of article).
Absorption Refrigeration Machines
In Germany, closed absorption refrigeration machines with liquid
sorbent (water-lithium bromide) are most often operated in combination
with heat and power generation (cogeneration) (i.e. with block unit
heating power plants, district heating), but can also be assisted
by vacuum tube solar collectors (operating temperature above 80
°C). With a single-step process the COP is 0.6-0.75, or up to
1.2 for a two-step process. A market overview is available from
the Consortium for Economical and Environmentally Friendly Energy
Use (Arbeitsgemeinschaft für sparsamen und umweltfreundlichen
Energieverbrauch (ASUE)).
Adsorption Refrigeration Machines
Closed processes with solid sorbents work with so-called adsorption
refrigeration machines (operating temperatures 60° - 95°;
COP = 0.3 - 0.7). Solar energy can easily be used in the form of
vacuum tube or flat plate collectors. A pilot system used for a
laboratory's climate control at the University Clinic of Freiburg
is fitted with tube collectors; the Fraunhofer ISE also took part
in its scientific conception. The refrigerating machine is composed
of two adsorbers, one an evaporator and the other a condenser. An
adsorber chamber takes up the water vapor, which is transformed
into the gas phase under low pressure and low temperatures (about
9°C) within the evaporator. Granulated silicate gel, well known
as an environmentally friendly drying agent, then accumulates it
(adsorbs the water vapor). In the other sorption chamber the water
vapor is set free again (the chamber is regenerated or "charged")
by the hot water from the solar collector (about 85°C). The
pressure increases and at the temperature of the surroundings (30°C)
the water vapor can be transformed once again into a fluid within
a cooling tower (condensed). Through a butterfly valve the water
is led back into the evaporator and the cycle begins from the beginning.
Both the condensed water (low temperature) and the sorption heat
(high temperature) are discharged.
Main components of the system at the
University Clinic of Freiburg: Adsorption refrigeration machine
(left) and solar thermal system (right).
The thermal operating power for this adsorption refrigeration machine
is produced by vacuum tube collectors with a surface area of 170
m². Additionally, heat storage tanks improve the use of the
solar heat. A cold storage tank functions as a buffer during short-term
demand fluctuations. During colder times of the year, the solar
energy heats the air inflow thereby reducing heating costs.
Sorption-Assisted Air Conditioning
Although the process of sorption-assisted air conditioning has
been known for a long time, it has only been used in Europe for
about 15 years. In principle, under middle European climate conditions,
sorption-assisted air conditioning systems can be operated everywhere
an air conditioner is wanted, for example in ventilation control
centers. Their economical operation is then possible if cost-effective
heat energy is available, i.e. from cogeneration plants, rather
than from over loaded district heating systems. New heat sources,
offering much promise, are solar thermal systems. Open sorption-assisted
air conditioning systems are fresh air systems, that is they dry
the outside air through sorption, pre-cool it with a heat reclamation
rotor and finally cool it to room temperature through evaporation-humidification.
The main principle of sorption-assisted air conditioning is shown
in the graphic. The solar energy is used to dehumidify the sorbent.
Basic structure of the process of
sorption-assisted air conditioning.
The most important steps of the process are:
1-2 Sorptive dehumidification of outside air with simultaneous
rise in temperature through the freed adsorption heat
2-3 Cooling of the air in the heat reclamation rotor in the countercurrent
to the exhaust air
3-4 further cooling of air through evaporation-humidification; the
air inflow to the building has a lower temperature and less water
vapor than the outside air
4-5 Heating of the air and if necessary addition of water vapor
5-6 Lowering of building's exhaust air temperature through evaporative
cooling in the humidifier
6-7 Heating of exhaust air in the countercurrent to the air inflow
in the heat reclamation rotor
7-8 Further heating of the exhaust air through external heat sources
(i.e. solar thermal system)
8-9 Regeneration of the sorption rotor through the desorption of
the bound water
At present, systems with rotating sorption wheels (sorption
rotors) are mostly in use. The sorption wheel has small air channels
that create a very large surface contact area, which has been treated
with a material that easily takes up moisture, such as silica gel.
The inflow air is dehumidified in one of the two sectors of the rotor
and heated through the adsorption process (the exhaust air serves
to dry the rotor). Finally, the inflowing air is cooled down in a
heat reclamation rotor. The heat transfer here is made possible through
the contact between the air and the rotor material. The last step
in cooling the inflowing air is with conventional evaporation humidification.
How well do Solar-Assisted Air Conditioning Systems
Operate?
Scientists of the Freiburg Fraunhofer Institute for Solar Energy
Systems ISE (Freiburger Fraunhofer Instituts für Solare Energiesysteme
ISE) tested solar assisted air conditioning systems for a study
of the International Energy Agency (IEA) in the context of the TASK
25 "Solar-Assisted Air Conditioning of Buildings". Detailed
descriptions and results of the compared systems can be gathered
from the study's conclusion [1]. Year simulations of five variants
of a solar-assisted system for air conditioning were conducted and
compared to a conventional system for different climates (Trapani/Sicily;
Freiburg and Coenhagen).
Energy Balance
Without the use of solar energy, thermally powered climate control
raised the primary energy use (thermal and electrical) for all of
the tested locations. The reason for this is the lower operating
numbers of this process in comparison to electrically powered compression
refrigeration machines.
Whether absorption or adsorption refrigerating machines are used,
a solar-covered share for cooling of 30 % (Freiburg) and almost
50 % (Trapani) is required to affect a primary energy savings. The
solar-covered share for cooling is the portion used for cooling
during the summer that comes from heat made available by the solar
thermal system. With coverage shares of up to 85 %, the primary
energy use can be decreased by over 50 % compared to the conventional
reference system. The results were ascertained from an example reference
office building and can therefore not simply be applied to other
cases or buildings.
In Trapani the sorption assisted air conditioning, in combination
with a compression refrigeration machine, led to a small primary
energy savings with a solar coverage share of 30 %. If the sun delivers
85 % of the heat for the air conditioning, then just about 50 %
of the primary energy can be saved. In this case there are two apparent
positive aspects: the sorption-assisted air conditioning can effectively
be used for air dehumidification and additionally it can achieve
relatively good overall efficiency.
Photo: Sorption-assisted air conditioning system in Portugal.
Cost Effectiveness
Although over 20 systems that use thermal solar energy to air condition
buildings and that can be technically and economically assessed
have been installed in Germany, there are still a number of obstacles
to be overcome when it comes to the implementation of solar-assisted
air conditioning. In the twelve countries taking part in the TASK
25 of the Solar and Heating Program of the IEA, experience with
about 30 systems has been gained and currently 10 systems are being
tested as a part of a demonstration program. Such pilot and demonstration
programs are still necessary so that cost reductions become possible
and so that relevant energy savings can be assured. Standardized
programs, matured concepts and the development of components are
starting points that can contribute to improved cost effectiveness
and wide applicability of solar-assisted air conditioning.
Because solar cooling is based on thermally driven processes instead
of the normal electrical cold production, the costs for the used
heat plays a central role: a fundamental problem arises from the
inherently higher costs of solar heat compared to heat energy produced
by fossil fuel systems or waste heat. Experts at the Fraunhofer
ISE expect no economical advantages of the solar air conditioning
in this respect. Their use becomes interesting if favorable requirements
for a high output of solar heat are present and if the system also
delivers energy for heating. The cost of electricity could also
pose an argument for solar cooling: The thermally powered cooling
process requires only a fourth (absorption/adsorption) or half (sorption-assisted
air conditioning) of the electrical power required by the conventional
reference system.
The ISEs comparative testing showed that during the process the
sorption-assisted air conditioning connected to a conventional machine
(compression refrigeration machine) represents the most promising
system combination, at least for a Mediterranean climate. The sorption-assisted
air conditioning produced the lowest costs at all locations, while
the adsorption machines were the most expensive solution. The scientists
at the Fraunhofer ISE see a chance for sorption-assisted air conditioning
in the cooperation between German facilities and companies that
have gained experience with the operation of sorption processes
for climate control and large solar thermal systems. Using this
know-how, especially in Mediterranean regions, a gap could be found
in the market.
Process
closed
open
Coolant circulation
closed refrigerant circulation systems
open refrigerant circulation systems (in
contact with the atmosphere)
Process baic principle
cold water production
air dehumidification and evaporative cooling
Sorbent type
solid
liquid
solid
liquid
Typical material systems (refrigerant/sorbent)
water- silica gel ammonia- salt*
water-water-lithium bromide, ammonia- water
water-silica gel
water- lithium chloride- cellulose
water- calcium chloride, waterlithium chloride
Marketable technoloy
adsorption refrigeration machine
absorption refrigeration machine
sorption assisted air conditioning
-
Marketable output [kW cooling]
adsorpzion refrigeration machine [50 - 430 kW]
absorption refrigeration machine: 35 kW - 5 MW
20 kW - 350 kW (per module)
-
Coefficient of Performance (COP)
0.3 - 0.7
0.6 - 0.75 (one step)
<1.2 (two step)
0.5 - >1
>1
Typical operating temp.
60 - 95°C
80 - 110°C (one step)
130 - 160°C (two step)
45 - 95°C
45 - 95°C
Solar technology
vacuum tube collector, flat plate collector
vaccum tube collector
flat plate collector, solar air collector
flat plate collector, solar air collector
*still in development
Table 1: Overview of processes for thermally powered cooling and
air conditioning
Material and Pictures: Fraunhofer ISE: Solarserver Editor: Rolf
Hug. We thank Dr. Hans-Martin Henning and Diplom Engineer Carsten
Hindenburg for their friendly support.
