Efficient use of waste heat and renewable heat sources

Efficient use of waste heat and renewable heat sources

1. Describe how you might recover heat from (a) a process exhaust gas stream (e.g. from an oven) and (b) a process warm water stream (e.g. from a commercial dishwasher). Discuss any aspects of the streams that may influence your choice of heat recovery system.

2. An industrial dryer operates for 60 hours/week, 50 weeks/year. It exhausts 200 kW/hr of heat and the value of the energy is 6p/kWh. A heat exchanger of 50% efficiency is put into the exhaust stream to recover a proportion of this heat. The installed cost of the heat exchanger is £22,500. The heat exchanger pressure drop needs a 2 kW fan to overcome it, and the cost of electricity is 10p/kWh

Calculate the simple payback period for the installation, taking into account the running costs, as well as the benefits.

3. A domestic air conditioning unit has a COPc of 2 Thinking of the refrigeration cycle and the inefficiencies in the various components, how could you attempt to increase the COP by modifying components?

4. The industrial dryer in Question 2 currently uses a heat exchanger for heat recovery. If the exhaust air is highly humid, as would be common on a dryer, there would be advantages in recovering latent heat as well as sensible heat. (Often you will find that the latent heat content is substantially greater than the sensible heat content).

Discuss how you might configure a heat pump which could recover the latent heat (and sensible heat) from the exhaust air then deliver it to the incoming fresh air. Is it possible now to recycle the exhaust air? If so, how would you reheat it?

5. Prime movers are being studied for powering combined heat & power (CHP) units in the home and in industry.

Sketch how the heat from these prime movers (e.g. a gas turbine, a small Diesel engine or another prime mover) can be recovered for (a) water heating, (b) providing chilled water (an outline of appropriate refrigerating equipment is required). 

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