UNIVERSITY OF PLYMOUTH
SCHOOL of ENGINEERING
 BENG3 THERMAL & FLUIDS ENGINEERING (THER305)
 TUTORIAL EXAMPLES in ENERGY ECONOMICS

1.       In seeking to use the energy required by a large office block as efficiently as possible, state briefly what actions you might take in respect of the following:­

          (i) Electricity usage.

          (ii) Heating and Cooling requirements. [actions]

2.       A single-storey flat-roofed factory building is 30m x 100m with a roof height of 3.8m and has the following thermal characteristics.

          Floor U-value 0.15 W/m2K;

Roof U-value 0.9 W/m2K;
Glazing Area 700 m2 U-value 5.6 W/m2K;
Wall U-value 1.1 W/m2K;

          Ventilation 3 air changes per hour.

          Assuming the heating load is offset by 40 kW available from lighting, machines etc. Estimate the heating requirement (in kW) of the building, assuming 19°C inside temperature and -1°C outside temperature.           [378 kW]

3.       A furnace used to produce expanded insulation has an exhaust gas temperature of 760°C and an oil consumption of 150 litres/h. A heat recovery scheme is envisaged whereby heat from the exhaust gases is used to pre-heat the combustion air. Because the exhaust gases are used to convey the product to a collection silo a simple tube-in-tube type heat-exchanger has to be used whose thermal ratio is likely to be only 0.22. The capital cost of the heat recovery modifications is going to be £20,000.        By determining the break-even time, comment on the value of the investment as an energy saving measure. Show a diagrammatic sketch of the proposal.

          The following data is relevant:
Relative density of fuel oil 0.82.
Lower calorific value of the fuel oil 42 MJ/kg
Cost of the fuel oil £140/tonne.

          Plant utilisation 2 x 8 hour shifts for 48 x 7 day weeks per annum.
Mean specific heat capacity of the exhaust products 1.05 kJ/kgK.
Assume the air/fuel ratio is sufficiently high to ignore the fuel mass flow in thermal calculations and ambient air temperature 15°C.  [ approximately 1 year]

          Solution is on two pages   Page1   Page2

4.       An office block discharges air from its air-conditioning system at 22°C during the heating season when the outside air temperature (average) is 4°C. In order to recover some of the waste heat a feasibility study is carried out on the following:­

          (i) A thermal wheel - Capital cost £1753, thermal ratio 0.89.

          (ii) Run-around heat-exchangers - Capital Cost £1102, thermal ratio 0.43.

       (iii) Heat pump installation (Average COP = 5.2, electrically driven) - Capital Cost £5000,
    thermal ratio 1.2.

          Assume that the incoming air quantity is 2.1 m3/s and the system operates under the above conditions for 10 hours/day over a heating season of 100 days, determine which of the above options is the most economically attractive. Assume the energy cost for air heating is 0.45 p/MJ and the energy cost for running the heat-pump is 2.9p/kWh.                                         [(i)]