Heat Pumps

Heat Pumps

Heat pumps operate on the refrigeration cycle (more precisely, on the vapor-compression refrigeration cycle). A schematic of the refrigeration cycle is shown as Fig. 5.13 in the text.

Just like the Rankine steam-cycle, the refrigeration cycle has a boiler (though it is now called an evaporator), a condenser, a pumping (or compression) process, and an expansion process. The following table shows the similarities and differences between the two cycles.

Process	Rankine Cycle	Refrigeration Cycle
Compression	Step 1. Water (in liquid phase) pumped from low to high pressure. Little change in temperature. Work is done on the working fluid.	Step 3. Vapor (ie, working fluid as vapor) compressed from low to high pressure. There is a significant increase in the temperature of the working fluid. (Note the difference.) Work is done on the working fluid. Note: per unit of mass compressed, it takes a lot more work to compress a vapor than a liquid.
Boiling/Evaporation	Step 2. The water is heated and converted to steam. Heat (Q_H) at high temp (T_H) is added to the working fluid. Heat flows from the hot space into the working fluid.	Step 2. The working fluid (as a mixture of liquid and vapor) is heated and converted to totally vapor. Heat (Q_C) at low temperature (T_C) is added to the working fluid. Note the key difference. Heat is flowing from the cold space into the working fluid.
Expansion	Step 3. The high pressure, high temperature steam expands through the turbine, changing to low pressure, low temperature steam (or to mixture of steam and liquid water). Work is done, that is, there is an output of work.	Step 1. Condensed (liquid) working fluid expands from high pressure and high pressure. The expansion occurs across a valve. The result is a mixture of liquid and vapor working fluid at low pressure and low temperature. No work is involved. Note the lack of work. This is a key difference.
Condensation	Step 4. Heat (Q_C) at low temperature (T_C) is rejected from the working fluid. The heat is carried away by the coolant - eg, sea water or cold water routed to a cooling tower. The result is liquid working fluid at low pressure and temperature.	Step 4. Heat (Q_H) at high temperature (T_H) is rejected from the working fluid. The result is liquid refrigerant at high pressure and relatively high temperature. Note the key difference. Heat is flowing from the working fluid to the warm space (for example, the kitchen).

The Rankine cycle is shown in Fig. 3.3. Marching around the cycle: 1(pump)® 2(boiler)® 3(turbine)® 4(condenser).

To march around the refrigeration cycle, we refer to Fig. 5.13. The process is:

1(expansion)® 2(evaporation)® 3(compression)® 4(condensation).

Note the pumping and expansion steps are exchanged in the two cycles. Note, heat is always added in the boiling/evaporation step, and heat is always rejected in the condensation step. However, the relative temperatures are exchanged. In the Rankine cycle, heat is added at high temperature and rejected at low temperature. In the refrigeration cycle, heat is added at low temperature and rejected at high temperature.

H₂O does not have the properties needed of the working fluid in the refrigeration cycle. Ammonia, Freon, and the new refrigerants with H added to Freon-type molecules are used.

The heat pump works by the refrigeration cycle. In cooling mode, that is, in "air conditioning" mode, the cool space is the built environment and the warm space is the outdoors. In heating mode, the cool space is the outdoors (or the ground) and the warm space is the built environment.

In cooling mode, we march around the cycle as follows (hx = heat exchanger):

1(expan valve)® 2(indoor hx)® 3(compressor)® 4(outdoor hx).

In heating mode, we march around the cycle as follows:

1(expan valve)® 2(outdoor hx)® 3(compressor)® 4(indoor hx).

In order to handle both modes, a switching system is used to send the working fluid (ie, the refrigerant) to the "correct" heat exchanger following the valve and compressor.

Some of the new heat pumps use the ground as the cool space. That is, the coils of the outdoor heat exchanger are buried (or fluid pumped from underground is used as Q_C). In heating mode, Q_H = Q_ground + W. In cooling mode, the "coolth" of the ground may be directly used to cool the interior space, saving electricity.

Resistance heating coils are also added. These are placed in the stream of air blown through the heat pump hx to the interior space. These coils only kick in when the heat pump cannot keep up with the heat demand. (ME Professor John Kramlich saved considerable electricity by maintaining a slightly higher thermostat setting on his heat pump on cold winter nights. The resistance coils, which gobble up a lot of electricity, didn’t kick in when the thermostat was moved to the daytime temperature setting in the morning.)

The efficiency of the heat pump is called the "coefficient of performance", or COP:

COP = "energy we want"/ "energy we pay for"

COP_{cooling mode} = "heat pulled out of cool space"/ "electrical energy for running compressor"

COP_{cooling mode}= Q_C/ W

COP_{heating mode} = "heat put into warm space"/ "electrical energy for running compressor"

COP_{heating mode} = Q_H/ W = (Q_C+W)/ W

For a real heat pump, W includes the electrical energy required to run the heat exchanger blowers (which is fairly small) and the resistance heater.

If a heat pump could create no net increase in the disorder of the universe, the COPs would be:

COP_{cooling mode}= Q_C/ W = Q_C/ (Q_H – Q_C) = 1/ (T_H/ T_c – 1)

COP_{heating mode} = Q_H/ W = Q_H/ (Q_H – Q_C) = 1/(1 – T_C/ T_H)

Of course, these are highly ideal COPs, but they serve to indicate important trends for real systems.

For a high COP in cooling mode, the warm space should not be too warm!

For a high COP in heating mode, the cool space should not be too cool!

Thus, Western Washington and Oregon, with relatively cool summers and relatively mild winters, appear to be good places for the heat pump. The ground, being of fairly even temperature, is a good source of Q_C for heating mode.