BENG2 APPLIED THERMODYNAMICS
(THER 205)
TUTORIAL EXAMPLES in ENERGY EFFICIENCY
1.
A domestic property measures 10m x 12m x 6m (height).
The U-values for the roof, walls, windows and floor are 0.24 W/m²K, 0.5 W/m²K ,
3.0 W/m²K & 0.17 W/m²K respectively. Find the steady state fabric heat loss
when the inside temperature is 22ºC and the outside is –1ºC. Assume the window
area is 20% of the total wall area. [7.20 kW] [spreadsheet
solution]
2.
Look up typical U-values for single, double and triple
glazing. If the cost of the windows goes up by 50% for each additional layer of
glazing determine which represents the most cost effective energy saving
measure. [soln]
3.
The property described in Q.1 has an average
ventilation rate of 0.5 air changes per hour. Determine the ventilation heat
loss. [2.846 kW]
4.
If the property described in Q.1 is heated by a
conventional gas boiler with an average operating efficiency of 64% and gas
costs 8p/kWh, determine the annual running cost of the heating system assuming
it operates at full rated output for 5 hours per day over a heating season of
100 days. [£627.86]
5.
In a compressed air system, the air is compressed to 8
bar gauge from SSL temperature and pressure. The electric motor driving the
compressor has an efficiency of 89%, and the compressor itself has a mechanical
efficiency of 86%. If the average mechanical efficiency of the air tools and
actuators which the system operates is 75% what is the overall energy
conversion efficiency. (Assume zero clearance volume in the compressor, and a
polytropic index (n) of compression and expansion of 1.3). [35.0%]
6.
Petrol and Diesel both have calorific values of 43.22
MJ/kg. If the average operating efficiency of a petrol engine is 26% and that
of a diesel engine is 39%, what is the fuel consumption (in litres per hour)
required to produce 25 kW. Assume the Relative Density of diesel is 0.820
and
that of petrol 0.740.
[10.82 litres/h petrol, 6.51 litres/h diesel]
7.
A petrol engined stand-by electrical power generator
has the following energy conversion efficiencies. Fuel to mechanical energy
27%, mechanical to electrical energy 81%. Its maximum rated output is 100 amps
at 240 volts.
If petrol costs 78 p/litre determine the electricity cost in p/kWh.
At half its maximum power output (50 amps) the petrol engine efficiency drops
to 15%, and the generator efficiency drops to 76%. Re-calculate the cost of
electrical energy and comment on the effect of part load operation.
Use the data for petrol given in Q.6. [40.2 p/kWh & 77.0
p/kWh respectively]