Page 115 - Low Temperature Energy Systems with Applications of Renewable Energy

104 Low-Temperature Energy Systems with Applications of Renewable Energy

3.3.2 Air heating systems

One of the main features of air heating systems is the lack of an intermediate heat-
transfer fluid or heat carrier. This has both positive and negative aspects. First, the
problem of “defrosting” the system disappears with its long-term shutdown in winter.
In addition, due to the high rate of hot air circulation, the inertia of an air system is
much lower than that of the water system, where the boiler first heats the heat carrier,
then the metal pipes and radiators, and - only later - the air indoors. The air heating
system can operate efficiently in the ventilation mode. It has a simpler design than
the traditional water system owing to the lack of a special boiler room, piping systems,
radiators, pumps and complex control automation. The main disadvantages of air heat-
ing are due to the movement of large volumes of air in the room which is heated. This
reduces comfort (drafts), leads to the movement of dust, and promotes the spread of
bacteria throughout the room being heated or ventilated. It is also necessary to take
into account that air ducts have a larger cross-sectional area than the internal pipe lines
of water heating systems, and therefore large-scale openings in the walls are required
for their installation [7]. But despite these disadvantages, air heating systems based on
a heat pump offer lower energy consumption, and they are environmentally friendly
since they operate without fuel burning on the premises, and therefore do not produce
harmful emissions into the on-site atmosphere.

3.3.2.1 Air heating using the heat of ambient air

The two simplest heat pump circuits for air heating using ambient air heat are consid-
ered below [6]:

1) Air heating without ventilation; a so-called “split system” with a remote (relative to heated
space) compressor, evaporator or condenser, operating in a heating or air-conditioning mode;
and
2) Air heating with room ventilation and a heat pump completely separate from the room being
heated.
The effectiveness of the first circuit, which is presented in Fig. 3.20, is due to the
insignificant heating of internal air in the HP condenser and the lack of heating external
ventilation air to the temperature in the room. However, taking heat out of ambient air
requires air cooling at the HP evaporator outlet to sufficiently low temperatures, which
worsens the working conditions of the heat pump, especially under conditions of low
ambient temperatures.
The operational efficiency of the second heating system, shown in Fig. 3.21, de-
pends, to a large extent, on the heating of external ventilation air from the ambient tem-
perature to the temperature at the entrance to the heated room, which can reach
significant values, especially at low ambient temperatures.
Moreover, in this system it is possible to use the heat of exhaust air that increases the
air temperature at the outlet of the HP evaporator and which improves the working con-
ditions of the HP. In this connection, in Ref. [7], the thermodynamic efficiency of the
above-mentioned heating circuits was determined and a comparison was made, showing
that these systems differ in their effectiveness from each other but only by a little.