Chapter 5, Problem 5.13P

5.12

A heat engine has a solar collector receiving 600 Btu/h per square foot, insidewhich a transfer medium is heated to 800 R. The collected energy powers aheat engine that rejects heat at 100 F. If the heat engine should deliver 8500Btu/h, what is the minimum size (area) of the solar collector?

The heat pump cycle shown in the figure below operates at steady state and provides energy by heat transfer at a rate of 15 kW to maintain a dwelling at 22°C when the outside temperature is Toutside 5°C. The manufacturer claims that the power input required for this operating condition is W cycle = 4 kW. 0 = i Is this cycle possible? Yes No QH = 15 kW bok and Modia Tinside = 22°C kW/K = Heat pump cycle Applying energy and entropy rate balances determine the rate of entropy production, in kW/K. Toutside W cycle

Ts diagrams for two reversible thermodynamic power cycles are shown in the following figure. Both cycles operate between a high temperature reservoir at 500 K and a low temperature reservoir at 300 K. The process on the left is the Carnot cycle described in Section 2.9. The process on the right is a Stirling cycle, which is similar to a Carnot cycle, except that the two steps (state 4 to state 1) and (state 2 to state 3) are at constant volume. Which cycle, if either, has a greater efficiency? Explain.

Two reversible cycles are in series, each process doing the same net work, Wcycle. The first cycle receives energy QH by heat transfer from a hot reservoir at 1000°R and energy Q is reinjected by heat transfer to a reservoir at an intermediate temperature, T. The second cycle receives energy Q by heat transfer from the reservoir at temperature T and reinjects the QC energy by heat transfer to the reservoir at a temperature of 400°R. All energy transferred is positive in the direction of the arrow. Determine: a) the intermediate temperature T, in °R, and the thermal efficiency for each of the two cycles; b) the thermal efficiency of a simple reversible cycle operating between the hot and cold reservoirs at 1000°R and 400°C, respectively. Then determine the net work done by the simple cycle, expressed in terms of the net work done by each of the two cycles, Wcycle.

Ts diagrams for two reversible thermodynamic power cycles are shown in the following fig- ure. Both cycles operate between a high temperature reservoir at 500 K and a low temperature reservoir at 300 K. The process on the left is the Carnot cycle described in Section 2.9. The process on the right is a Brayton cycle, which is similar to a Carnot cycle, except that the two steps (state 1 to state 2) and (state 3 to state 4) are at constant pressure. Which cycle, if either, has a greater efficiency? Explain.

5.1

One kg of fluid expands reversibly according to a linear law from 4.2 bar to 1.4 bar. The initial and final volume are 0.004 and 0.02 m² respectively. The fluid then cooled reversibly at constant pressure, and finally compressed reversibly according to a law PV=C back to initial condition of 4.2 bar and 0.004 m². Calculate the work done for each process, the net work done and sketch the cycle on the P-V diagram. (4480, -1120, -1845 &1515 N-m)

6.107 Figure P6.107 provides the schematic of a heat pump using Refrigerant 134a as the working fluid, together with steady-state data at key points. The mass flow rate of the refrigerant is 7 kg/min, and the power input to the compressor is 5.17 kW. (a) Determine the co- efficient of performance for the heat pump. (b) If the valve were re- placed by a turbine, power could be produced, thereby reducing the power requirement of the heat pump system. Would you recommend this power-saving measure? Explain. She P2 = P3 = 9 bar Tz = 60°C Saturated liquid Condenser Expansion W = 5.17 kW Compressor valve Evaporator m= 7 kg/min P1 =P4 = 2.4 bar FIGURE P6.107