An absorption cycle with integral refregerant storge

1. AN ABSORPTION CYCLE WITH
INTEGRAL REFREGERANT
STORAGE

2. INTRODUCTION
• The present energy crisis has created renewed interest in
absorption cycle system for efficient heating/cooling of
buildings
• A cost effective system for efficient system is yet to be
determined
• In continuous operation of these systems heating /
cooling is accomplished using low grade system and
auxiliary system is also needed during off peak hours.
• Energy storage is an essential component for these
systems.

3. INTRODUCTION (Contd.)
• A low grade heat based energy storage energy system for
environment control would be more cost effective if the
same unit is used for both heating and cooling of a building
and an efficient energy storage is associated with it.
• Conventional solar energy storage methods like sensible
heat in water and rocks have low storage capacity per unit
volume , high thermal loss to ambient and are bulky in
nature.
• Two possible locations for solar energy storage in
absorption cycle heat pumps for space cooling are on the
input side and output side

4. INTRODUCTION (Contd.)
• On input side as hot storage between low grade heat source and
generator of the heat pump
• On output side as cold storage between the evaporator and the
space to be cooled.
• On the input side the possible temperature change of 20-25°C
produces a storage capacity of 100 MJ/m3 for water and
35MJ/m for rocks (with void fraction of 0.33)
• Thermal losses from such storage are substantial due to the
temperature difference of 50-60°C from mean storage
temperature to the ambient temperature.
• As the storage size is increased relative to other system
components, each new increment of thermal capacity is less
effective than previous ones

5. INTRODUCTION (Contd.)
• On out put side case of chilled water storage, an effective
temperature change of 7-10°C would limit the storage
capacity to 40 MJ/m3.
• The thermal losses could be low since the temperature
difference of 15-20°C between the average storage
temperature and the ambient is less than half of that for
storage on the input side.
• With the cold storage only, the refrigerator/air conditioner
must be sized to the peak energy input resulting in low
utilization of installed plant capacity and low conversion
efficiency at part load operation of the system.

6. Proposed solar space cooling system with energy storage options

7. INTRODUCTION (Contd.)
• Phase change storage media with constant storage
temperature can provide a means of reducing the storage
volume ( at least by a factor of 20-25)
• Some practical difficulties include.
1. performance degradation on cyclic operations of the storage
system.
2. poor stability due to an incongruent fusion (salt hydrates) and
due to decompositions (in usual paraffins containing several
hydrocarbons).
3. the need for large heat exchangers due to the poor thermal
conductivity of the medium

9. BASIC OPERATION OF ABSORPTION
CYCLE WITH REFRIGERANT
STORAGE

10. • Solar air conditioning with refrigerant storage has been
investigated in detail and shown to be technically feasible.
• Space heating with energy storage in aqua-ammonia
absorption cycle heat pump has not been evaluated so far.
• The space heating systems are most cost effective with
higher market demand in cold countries as compared to
space cooling systems.
• The absorption cycle heat pump with possible energy
storage for space heating application is shown in the
figures 6.2(a) and (b).
• The system includes the generator, rectifier, condenser
evaporator and absorber along with preheating/sub -
cooling heat exchangers, pump and pressure reducing
valve.

11. Fig 6.2 (a)

12. Fig 6.2 (b) Absorption heat pump system

13. • In absorption systems, a storage volume is provided in
association with the con denser to store the refrigerant
during the hours of excess generation.
• The stored refrigerant is released in a manner to meet the
required load during the off-generation hours.
• Storage is also needed in the absorber to accommodate
sufficient absorbent to keep concentration with in
allowable limits.
• The condenser storage is assumed to have a regulating
valve at its outlet to control refrigerant flow.
• Heat input to the generator is supplied through a flat plate
solar collector
• The heat of condensation at the condenser and the heat of
solution at the absorber are rejected to the atmosphere by
a cooling tower ( where water circulates through the
absorber store, absorber, condenser store and condenser)

14. • Room air is circulated through the evaporator at constant
inlet temperature.
• Based on the component heat exchanger models
(describing heat exchangers in terms of log mean
temperature difference) and typical heating/cooling loads
for specific locations.
• The basic principle of an absorption cycle heat pump is
similar to the absorption cooling cycle except that the role
of the heated air and cold air is reversed.
• The choice of the refrigerant-absorbent pair to be used for
an absorption cycle with refrigerant storage is somewhat
different from that for a conventional absorption
refrigeration cycle

15. • The refrigerant-absorbent combination should be such for
which
I. The absorption capacity of the absorbent is large
II. The specific heats of the absorbent and the refrigerant are
low.
III. The heat of exothermic reaction is large
IV. Vapor pressure of the absorbent at the genera ting
temperature is low
V. Vapor pressure of the refrigerant at the working
temperature is low
VI. Latent heat of vaporization of the refrigerant is large.

16. • A refrigerant with low vapour pressure is preferable as a
large volume of storage must be provided at the high
pressure level of the system.
• High latent heat of the refrigerant is desired to reduce the
storage volume for the liquid refrigerant
• Aqua ammonia mixture - low storage volume but high
storage pressure ,high pumping power and high pressure
tankages.
• The sensible heat contained in the hot weak solution is
used to heat the strong solution entering the generator
through solution heat exchanger (preheater) ,thus reduces
the amount of energy input to be supplied to the generator.
• This may greatly reduce the high pressure requirement.

17. • A sub-cooler used between the condenser and the
evaporator increases the amount of energy extracted by
the evaporator from the ambient air.
• A further refinement to improve the performance of an
absorption cycle is to utilize this heat (rejected from
rectifier) in raising the temperature of the strong solution
before entering the generator.
• This also reduces the generator heat input requirement of
the system.
• The refrigerant vapour is mixed with water vapour which
degrades the performance of the system.
• A complex rectification system consisting of a distillation
column, analyser and a separator is required to separate
the water content from the refrigerant vapour.