Page 5 - Defrosting for Air Source Heat Pump

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Introduction

1.1 Background

Similar to a water pump that could move fluids (liquids or gases) or sometimes slurries
against gravity, a heat pump can transfer heat from the heat source at a low temper-
ature to a heat sink at a high temperature by consuming power, which is against the
direction of heat transfer from high temperature to low temperature. In applications,
heat pumps are widely used to transfer thermal energy in the opposite direction of
spontaneous heat transfer by absorbing heat from a cold space and releasing it to a
warmer one. The most common design of a heat pump includes four basic compo-
nents: a condenser, an expansion valve, an evaporator, and a compressor. Refrigerant
is the heat transfer medium being circulated through these components in a heat pump.
There are two main types of heat pumps: absorption and compression. An absorp-
tion heat pump could be powered by oil, gas, or solar energy. The fuel utilization effi-
ciency in an absorption heat pump is evaluated by the ratio of the energy supplied to
that consumed. For example, when gas is used to power an absorption heat pump, the
utilization efficiency could reach 1.0–1.5. Compression heat pumps, on the other
hand, are most likely powered by electricity. When comparing the performance of
a compression heat pump, the term “coefficient of performance (COP)” as the ratio
of useful heat to work input is commonly used. For a compression-type heat pump,
although it has the obvious advantage of higher energy performance than that of an
absorption-type heat pump, its disadvantage of consuming high-quality electricity
limits its application in waste energy recovery.
Heat pumps might also be categorized by their heat sources, such as air, ground
soil, and water, into air-source heat pumps (ASHPs), ground-source heat pumps
(GSHPs), and water-source heat pumps (WSHPs). Clearly, air is everywhere, and
is the most common, safe, stable, and cheapest thermal source. An ASHP extracts heat
from the ambient air and transfers the heat to the indoor air as an air-air heat pump, or
to hot water in a domestic hot water tank as an air-water heat pump. In practice, air-air
heat pumps are widely found; science air conditioners just work following the same
principle. When an air conditioner is used in summer, it removes thermal energy from
the indoor air to the outdoor air. However, when it reversely operates in winter, ther-
mal energy is moved from the outside air for heating the indoor air. ASHPs are the
most common type of heat pumps with the lowest investment cost. Air-water heat
pumps may also easily be found as water heaters, by transferring the extracted heat
from the outside air for water heating all year round. In winter, the hot water provided
by an air-water heat pump may also be partly or wholly used in a water-based space-
heating system, such as a floor radiant heating system, which is efficient and enables a
comfortable indoor environment. ASHPs may also be used to extract thermal energy
from exhaust air, and they are thus sometimes called exhaust air heat pumps. The
exhaust air can be from buildings due to ventilation or from some industrial
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00001-1
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