ISAT 413 - Module III: Building Energy Efficiency

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ISAT 413 - Module III Building Energy Efficiency
Topic 9 Advanced Heat Pumps

• Heat Pump Water Heater
• Heat Recovery from a Swimming Pool Using a Heat
  Pump
• Mechanical Vapor Re-Compression
• Demonstration of a 2-RT Heat Pump

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• Heat Pump Water Heater Ground Coupled

A small nursing home is to be heated by a
low-thermal mass under-floor hot water heating
system. Two ways of providing the heat input are
being considered. (a) An electric heat pump with
a low temperature source provided by ground
water the ground water is pump to evaporator and
returned by gravity to a rubble-filled soak way.
(b) A gas-fired condensing boiler. The required
design heating load is 100 kW and the under-floor
system has an average water temperature of 35oC.
The annual hours of use of the heating system at
the average conditions may be taken as 4000.
Using the data below, neglecting heat losses and
pumping power, make an initial estimate of the
time after which the heat pump system will start
to make a net saving neglecting all annual costs
other than the fuel costs.
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Ground Coupled HPWH (continued)
Data
Heat Pump Vapor compression with refrigerant
R-12 Minimum Temperature Difference for HXs 10
K Overall Compressor Efficiency 0.86 Average
Temperature of Ground Water 10oC Compressor
Inlet Dry Saturated Vapor Condenser Sub-cooling
0 K Compressor Isentropic efficiency
1.0 Capital Cost of the Complete System
L21,000 Cost of Electricity 3.7 p/kW Condensing
Boiler Efficiency 90 capital Cost of Boiler
L5,500 Cost of gas 1.2 p/kW
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Ground Coupled HPWH (continued)
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Ground Coupled HPWH (continued)
Heating load energy balance Q_dot_h100 Q_dot_
hm_dot_r(h_2-h_3) Q_dot_cm_dot_r(h_1-h_4) W_do
t_isnQ_dot_h-Q_dot_c eta0.86 W_dot_cW_dot_isn/e
ta Savings C_run_hpW_dot_c3.7/100 C_run_Boiler
(Q_dot_h1.2/100)/0.9 21000C_run_hpt5500C_run
_Boilert yt/(4000)
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Ground Coupled HPWH (continued)
Solutions C_run_Boiler1.333 EP/hr C_run_hp0
.737 EP/hr h_1187.53 kJ/kg h_2209.81
kJ/kg h_379.70 kJ/kg h_479.70
kJ/kg m_dot_r0.76853 kg/s P_13.083
bar P_210.835 bar P_310.835
bar Q_dot_c83 W Q_dot_h100 W s_10.696
kJ/kg-K s_20.696 kJ/kg-K t25991
hr T_10.00 C T_345.00 C W_dot_c20
W W_dot_isn17 W y6.50 yr
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• Heat Recovery from a Swimming Pool Using a Heat
  Pump

8

• Mechanical Vapor Re-Compression

• The use of heat pump for heat recovery in cases
  where vapor is continuously evaporated is
  becoming well-established. The vapor evaporated
  from the process is compressed to a higher
  pressure and then condensed providing a heating
  effect the system therefore is known as
  Mechanical Vapor Re-compression, MVR. Examples of
  MVR applied to evaporation, distillation, and
  drying are shown below

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An Example of MVR (Mechanical Vapor
Re-Compression)

• A rotary steam drier similar to that shown in
  the following Figure operates with 1 kg/s of
  steam entering the drier at 6 bar and condensing
  with no undercooling. The condensate is throttled
  to 0.7 bar before passing through the heat
  exchanger where it is evaporated leaving as a dry
  saturated vapor. Assuming that the isentropic
  efficiency of the steam compressor is 0.65 and
  that the combined mechanical and electrical
  efficiency is 0.9, calculate (i) the overall
  coefficient of performance of the heat pump (ii)
  the rate of cost saving compared with providing
  the heat for the drier with a gas boiler of 80
  efficiency take the cost of electricity as 4
  p/kWh and of gas as 1.3 p/kW-h.

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An Example of MVR (continued)
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An Example of MVR (continued)
Power required and system COP W_cm_dot_s(h_2-h
_1) W_mechW_c/0.9 Combined mechanical and
electrical efficiency is 0.9 COP_MVRQ_dot_h/W_me
ch Costs/Savings C_run_MVRW_mech4/100 C_run_bo
iler(Q_dot_h/0.8)1.3/100 Gas boiler
efficiency is 80 S_(C_run_boiler-C_run_MVR)/C_
run_boiler100
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An Example of MVR (conclusion)
Solutions COP_MVR3.402 C_run_boiler43.945
NP/hr C_run_MVR31.795 NP/hr eta_isn0.650 h
_12660 kJ/kg h_23375 kJ/kg h_2s3125
kJ/kg h_3670.618 kJ/kg m_dot_s1.0
kg/s P_10.7 bar P_26.0 bar Q_dot_h2704.
3 kW S_27.6 s_17.479 kJ/kg-K s_2s7.
479 kJ/kg-K W_c715.4 kW W_mech794.9 kW