Refrigeration Storage

1. A NOVEL LATENT HEAT STORAGE FOR SOLAR SPACE
HEATING SYSTEMS: REFRIGERANT STORAGE

2.  Introduction
• Solar absorption refrigeration systems 1 - 3 used for comfort cooling of buildings
are gaining in popularity since the energy crisis. Because solar input is variable and
intermittent and the demand for heating/cooling is time dependent, energy storage
is an essential part of any solar energy system for space heating/cooling.
• A solar absorption refrigerator is a heat operated machine which includes a
generator, a condenser, an evaporator and an absorber together with a heat
exchanger, a mechanical pump and an expansion valve (Fig. 1).
• In the absorber, gaseous refrigerant from the evaporator is dissolved in an
absorbent liquid with a strong affinity for the gas and a low concentration of it.
• After the absorber, this liquid, now rich in refrigerant, is pumped to a generator at
the high pressure of the system and heated by solar energy to produce superheated
refrigerant gas and a solution, weak in refrigerant. The latter is returned to the
absorber to complete its cycle while the former flows to the condenser where it is
liquefied by ambient cooling.

3. Fig. 1. Basic solar absorption refrigeration system.

4. • After the absorber, this liquid, now rich in refrigerant, is pumped to a
generator at the high pressure of the system and heated by solar energy
to produce superheated refrigerant gas and a solution, weak in
refrigerant. The latter is returned to the absorber to complete its cycle
while the former flows to the condenser where it is liquefied by ambient
cooling.
• After passing through an expansion valve, the fluid is evaporated at the
low pressure of the system by withdrawing heat from the room air. The
resulting gas goes to the absorber to complete its cycle.
• Since the absorption process is exothermic, heat must be removed from
the absorber to keep its temperature low and ensure continued
absorption of the incoming gas.

5.  Novel Latent Heat Storage
• In solar absorption refrigeration systems, the amount of refrigerant
generated is dependent on the heat transferred to the generator from the
solar collector and hence on the solar insolation. Thus, the amount of
refrigerant generated tends to reach a peak at about noon when the solar
flux is high. However, due to the thermal inertia of the building structure,
the cooling load reaches a peak some time afterwards.
• In winter, the heating load reaches a peak during the night. Thus, for
year-round air-conditioning, some form of heat/cold storage is always
necessary to compensate for the phase difference between energy input
and demand.

6.  Conventional Storage Systems
• Solar energy can be stored as sensible heat in water or rocks; however, the storage capacity per unit
volume of these systems is limited by the specific heat of the storage material and the relatively small
temperature differences available in building applications. Although the rock pile system is a re-
generative heat exchanger and promises a good effectiveness, the energy recovery at usual
temperatures is not as high as is desirable. Moreover, the storage systems are bulky and have a poor
thermal response.
• Recently, some latent heat storage systems, involving phase change in solids, have been proposed and
investigated for low temperature thermal storage in solar energy applications; however, even for these
systems there are some major drawbacks, as follows.
(i) Loss of performance on recycling, due to separation of components in the case of salt hydrates
and to decomposition in the case of some paraffins.
(ii) The poor thermal conductivity of the medium necessitates a large contact surface with the heat
exchange fluid and increases the gross storage volume.
(iii) The systems are expensive.
• Thus, at present, although solar energy is cost competitive with other energy sources for space
heating/cooling of buildings, there are practical difficulties with available means of thermal energy
storage. A storage system with improved performance could enhance the viability of solar heating and
cooling systems.

7.  Concept of Refrigerant Storage
• For cooling applications using solar absorption refrigeration systems, the concept of
refrigerant storage is basically to provide, in association with the condenser, a
storage volume where the refrigerant can be accumulated during the hours of high
solar insolation.
• The stored liquid refrigerant is released into the evaporator as necessary to satisfy
the cooling load. Storage is also needed in the absorber to store not only the
refrigerant but also sufficient absorbent to keep the concentration within allowable
limits (Fig. 2).
• By careful matching of the system components, operational advantages, such as
early start up with low concentration solution and use of high concentrations when
the highest temperatures are available, can produce an improved daily performance.
A suitable working fluid for solar space cooling systems is lithium bromide-water
solution.

8. Fig. 2. Solar absorption refrigeration system for space cooling with refrigerant storage

9. • The calculated energy transfer rates for various components of the refrigeration system
(QG-generator heat input; Qc condenser heat output; QE--evaporator heat input and the
desired QB—building cooling load) are shown in Fig. 3.
Fig. 3. Hourly variation of heat transfer rate in various
components of the cooling system with a given building load.
It is seen that generation starts early in
the morning as a result of the low
concentration. For a short period after
starting, the refrigerant generated is
nearly all required to meet the building
load. Thus the storage rate is small, as is
the concentration change

10. • The system temperatures (Tt -generator temperature; TA absorber temperature and Te-
evaporator temperature) are shown in Fig. 4.
Fig. 4. Hourly variation of the system
temperature with a given building load
The evaporator temperature continues to rise
until the building demand starts to increase
after 10.00h (Fig. 4). At that time, the
combination of generator temperature and
absorber temperature allows the evaporator
temperature to decrease, with the result that
the load capability of the evaporator is
increased. The storage rate also increases from
the low value at the commencement of
generation.

11. • To understand the operation of refrigerant storage, the variation of the refrigerant
mass in the store is given as a function of time (Fig. 5); the variation of the incident
solar radiation intensity has also been depicted in the fig. below
Fig. 5. Hourly variation of mass of refrigerant in
store and solar radiation intensity incident on the
collector with a given building load.
It can be seen that generation of
refrigerant ceases several hours before
sunset although a significant amount of
energy is still being collected; the stoppage
occurs because of the high boiling point of
the solution which has become highly
concentrated with so much refrigerant in
the store. During the night, as the
refrigerant flows from the store to the
absorber, the evaporator cooling rate
continually decreases as the solution
concentration decreases and causes a high
pressure and temperature in the
evaporator.

12.  Concept of Solar Heat Pump Operation with Refrigerant
Storage
• As the heating load of a building is out of phase with the solar energy input,
generally larger storage capacities will be required than for equivalent space cooling
systems.
• Storage in solar space heating systems can be improved by using the concept of a
solar powered absorption heat pump. Because the absorber temperature in an
absorption heat pump system can be as high as 50 °C and heat must be removed
from the absorber to keep the temperature (and the corresponding refrigerant
vapour pressure) low, it appeared to the authors that the heat pump system with
refrigerant storage was worth investigating for winter heating.
• In the storage mode, refrigerant will be boiled from solution by solar energy in the
daytime and condensed for storage in the condenser store as in the cooling cycle.
• When space heating is needed, the condensed refrigerant will be evaporated in an
outdoor air or water coil before being dissolved back into the solution in the
absorber.

13. • The heat of solution in the absorber will be the energy source for space heating. Thus
it is possible to utilise the latent heat of the refrigerant solution as internal storage in
a solar absorption heat-pump system (Fig. 6).
Fig. 6. Solar absorption refrigeration system
for space heating with refrigerant storage.
A suitable absorbent-refrigerant combination
for solar heating may be water ammonia. A
first approximation of heat transfer rates in
the proposed system can be made by steady
state-thermodynamic analysis. The basic
mass balance and energy balance equations
for the various components of the system can
be solved to give a numerical estimate of the
rates of heat transfer and system
temperatures for winter heating conditions
and given building load. However, a more
dynamic transient model to optimise the
system parameters is needed in the realistic
situation.

14.  Solar Absorption Refrigeration System’s Advantages
(over other storage)
The concept of refrigerant storage in solar absorption refrigeration systems is
fundamentally sound and has the following advantages over other storage systems:
(i) The energy storage per unit volume is large as the latent heat of vaporisation of the
refrigerant/absorbent is high.
(ii) The refrigerant is stored at near ambient temperatures where heat losses from the
storage are minimal.
(iii) A further advantage is achieved in the lithium-bromide-water cycle that the storage
pressure is low so that the strength of the storage vessel is not critical.
While the LiBr-H20 system has some advantages for cooling, it is possible that other
combinations may be more successful for a heating system or where combined heating and
cooling is needed. If the system can be extended for year-round operation--cooling in summer
and heating in winter--there is a better chance of economic viability as a greater amount of
useful energy will be produced from substantially the same investment.