Page 55 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 43

13. Explain schemes of heat pumps with vapor-compression systems.
14. Sketch diagrams of absorption thermal installations.
15. Give examples of schematic solutions for industrial applications of heat pumps.
16. Describe systems of storage and transportation of heat.
17. Give two examples of systems with a thermal energy accumulator and compare them.
18. What are the advantages of using heat pumps for heating residential and industrial
premises?
19. Explain the principle of vapor-compression heat pump operation.
20. Explain the main purpose of each element of a vapor-compression heat pump.
21. Show several operating modes of heat pumps.
22. What heat sources are used for a heat pump?
23. Describe some technological systems of heat pumps for different sources of heat.
24. What heat-carriers are used in the heat supply pipeline with a heat pump?
25. What parameter gives the efficiency of heat pump operation and how is it defined?

Exercises

1. Prove in general that a system operating as a heat pump has a COP HP that is greater than the
COP R as a refrigerator by exactly one, namely, that COP HP ¼ COP R þ1, provided that the
unit operates with the same set of heat reservoirs.
2. Consider a basic vapor-compression heat pump (see Figs. 1.11 and 1.12) with the following
specifications:
Cycle working fluid, refrigerant R134a
Heating capacity, 500 kWt.
Compressor efficiency, 85%

R134a evaporates at 5 C.
Solve the following parts for two cases of different condensing temperatures; Case (a):

80 C and Case (b): 60 C, and discuss the differences.
Part A. Determine the state-point properties; use Table 1.1 as a template.
Part B. Calculate the mass flow rate of the R134a, kg/s.
Part C. Calculate the compressor power if the electric drive motor is 92% efficient, kW.
Part D. Calculate the heat pump COP HP .
Part E. Calculate the ideal Carnot heat pump COP C if it operated over the same temperature
limits as the actual heat pump.
3. Refer to Problem 2. Assuming all the heat released from the working fluid in the
desuperheater-condenser is delivered to water that circulates between the condenser and
the space to be heated, calculate the efficiency of exergy transfer from the heat pump to
the water if the water return temperature is 35 C and the pinch-point temperature difference

between the R134a and the water is 5 C. Do the calculations for both Case (a) and (b). The

dead-state temperature is 2 C (275.15 K) and the pressure is 0.1 MPa.

4. Refer to Problem 2, Case (a). Explore the feasibility of replacing the throttle valve with an
expansion machine. The saturated liquid at state 4 will be sent to a device that can handle
a mixture of liquid and vapor (such as a helical screw expander) to generate electrical power.
That power can be used to offset part of the input needed to drive the compressor.
Part A. Determine the power developed by the device if the machine has an isentropic effi-
ciency of 60% and its generator is 93% efficient.
Part B. Calculate the COP HP for this mode of operation.