(204z) Lithium Bromide-Water Absorption Heat Pump System for Heating Up Air From Waste Heat

The efforts for
energy conservation are requested because of the limitation of fossil fuel and
the environmental issue of global warming. Enormous amount of heat at lower
temperatures than 100 °C is wasted daily from industrial plants in the world-wide.
The petrochemical and steel industries are especially the largest energy-consuming
manufacturing industries. Waste hot water in the range of 60-90 °C is drained
from these industrial processes in large quantities. Absorption heat pump (AHP)
is known as one of the refrigeration techniques working by heat without using
compressor, but have been recently applied to steam generation or heating up
higher temperature by recovering waste heat as well.

In this study, an
innovative AHP system is suggested to heat up the temperature from waste heat
in a level of 80 °C and to produce hot air over 120 °C available for drying. Air
is heated up directly by heat exchange in the absorber working in the heating
mode of AHP. The main structure of the present AHP systemwas composed of an evaporator,
an absorber, a regenerator, a condenser, a solution heat exchanger, solution
pumps, and water pumps. The evaporator, the absorber and the regenerator
consisted of 74, 46 and 42 vertical spiral tubes made of copper with 23 mm in inner
diameter, respectively. Their length was 4848mm, 4940mm and 4352mm. In the
condenser, a bundle of 91 U-shaped bare- copper tubes of 16.6 mm inner diameter
was installed, and the total length was about 5500mm. The absorbent used in the
AHP was an aqueous solution of lithium bromide (LiBr) and the additives were
added for corrosion resistance. All operation process proceeded in a closed
system under sealed evacuated atmosphere.

The absorbent
solution condensed in the regenerator was fed to the absorber by solution pump through
solution heat exchanger. A film of the solution flowing down along the inside
wall of the vertical tubes was formed in the absorber, to absorb water vapor,
which was evaporated from a pure water film flowing on the inside wall of the
tubes heated by hot water at 80 °C , which flowed around the tube in the evaporator.
Then the solution was heated up during flowing down due to gaining exothermic
dilution heat of the solution and the latent heat by water vapor absorption. Air
streamed in counter flow around the tubes in the absorber and heat exchange
took place to heat up the air through the tubes from the solution. The diluted
absorbent solution by absorbing water vapor was returned into the regenerator
through the solution heat exchanger. In the regenerator, the absorbent solution
flowed down forming a film on the inside wall of the tubes, and was heated by
hot water at 80 °C, which flowed around the tubes. The solution was condensed
during flowing down by evaporation of water vapor. The evaporated water vapor transferred
into the condenser, and was condensed on the surface of the tubes which was
cooled by water fed into the inside. The water in the condenser was moved to
the evaporator.

The examination was
performed continuously on the performance of the AHP. When it was confirmed
that the operation achieved sufficiently a steady state, temperatures of the
fluids were measured at the inlet and outlet of each device. The absorbent
solution was sampled at the inlet and outlet of the absorber and the regenerator
to determine the concentration of LiBr using the infrared moisture analyzer
(Shimadzu MOC-120H).

The representative
results of the present AHP performance were indicated in Fig. 1. It was
resulted in that the temperature of hot air at the outlet of the absorber
achieved at above 120 °C by recovering heat of hot water at 80 °C. The
coefficient of performance, which is defined by the ratio of heat generated to
the power consumed for pumps of fluid flow, exceeds 20.

The present authors
acknowledge that this work was supported financially by Research and
Development Program for Innovative Energy Efficiency Technology under the New
Energy and Industrial Technology Development Organization (NEDO) project based
on a grant from METI.