an assessment of a solar-powered organic rankine cycle...
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James Freeman, Klaus Hellgardt, Christos N. Markides
AN ASSESSMENT OF A SOLAR-POWERED
ORGANIC RANKINE CYCLE SYSTEM FOR
COMBINED HEATING AND POWER IN
DOMESTIC APPLICATIONS
SusTEM Special Sessions
on
Thermal Energy Management
Department of Chemical Engineering,
Imperial College London, UK
Background
Organic Rankine Cycle (ORC) is a
proven technology for the generation
of power from low grade solar and
waste heat sources.
Challenges exist to develop this
technology for the small (domestic)
scale, and as part of an integrated
system for the provision of combined
heat and power.
Further specific challenges are
presented by the nature of the UK
climate and solar resource.
• To develop a techno-economic model of a domestic (small-scale)
combined solar heating and power (CSHP) system featuring an organic
Rankine cycle (ORC) with a generalised positive displacement
expander.
• To model the performance of the system in the UK climate over an
annual period.
• To evaluate the levelised cost of electricity produced over the system
lifetime, the additional thermal energy produced, and the potential for
CO2 emissions reduction.
Objectives
Overview of system model
Overview of system model
1 2
3 4
ORC sub-model
1 2
3 4
PUMP
• Fixed isentropic
efficiency. 𝜂Pump = ℎ2s − ℎ1 ℎ2 − ℎ1
ORC sub-model
EVAPORATOR
• Fixed pinch ΔT
at outlet.
1 2
3 4
PUMP
• Fixed isentropic
efficiency. 𝜂Pump = ℎ2s − ℎ1 ℎ2 − ℎ1
𝑇3 = 𝑇hs−in − ∆𝑇pinch
ORC sub-model
EXPANDER
• Generalised positive
displacement.
• Fixed isentropic
efficiency. EVAPORATOR
• Fixed pinch ΔT
at outlet.
1 2
3 4
PUMP
• Fixed isentropic
efficiency. 𝜂Pump = ℎ2s − ℎ1 ℎ2 − ℎ1
𝑇3 = 𝑇hs−in − ∆𝑇pinch
𝜂Expander = ℎ3 − ℎ4 ℎ3 − ℎ4s
ORC sub-model
EXPANDER
• Generalised positive
displacement.
• Fixed isentropic
efficiency. EVAPORATOR
• Fixed pinch ΔT
at outlet.
CONDENSER
• Water cooling
medium.
• Assumed at fixed
temperature.
1 2
3 4
PUMP
• Fixed isentropic
efficiency. 𝜂Pump = ℎ2s − ℎ1 ℎ2 − ℎ1
𝑇3 = 𝑇hs−in − ∆𝑇pinch
𝜂Expander = ℎ3 − ℎ4 ℎ3 − ℎ4s
𝑇cw−in = 10°C
ORC sub-model
EXPANDER
• Generalised positive
displacement.
• Fixed isentropic
efficiency. EVAPORATOR
• Fixed pinch ΔT
at outlet.
CONDENSER
• Water cooling
medium.
• Assumed at fixed
temperature.
1 2
3 4
PUMP
• Fixed isentropic
efficiency.
WORKING FLUID
R245fa
𝜂Pump = ℎ2s − ℎ1 ℎ2 − ℎ1
𝑇3 = 𝑇hs−in − ∆𝑇pinch
𝜂Expander = ℎ3 − ℎ4 ℎ3 − ℎ4s
𝑇cw−in = 10°C
ORC sub-model
𝜂collector = 𝑐0 − 𝑐1 ∙𝑇 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 − 𝑇ambient
𝐼𝑠𝑜𝑙ar− 𝑐2 ∙
𝑇 collector − 𝑇𝑎𝑚𝑏𝑖𝑒𝑛𝑡2
𝐼𝑠𝑜𝑙ar
𝑇ambient
𝑇outlet 𝑇inlet
𝐼solar
𝜂𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 ∙ 𝐼𝑠𝑜𝑙𝑎𝑟 ∙ 𝐴collector = 𝑚 fluid ∙ 𝑐𝑝∙ 𝑇outlet − 𝑇in𝑙𝑒𝑡
𝑚 fluid
𝑇 collector = 𝑇outlet + 𝑇inlet /2
𝐴collector = 15 m2 (DECC)
Solar collector sub-model
Assume quasi-equilibrium:
Non-concentrating collector
• Evacuated tube technology.
• Non-tracking.
• Model using global irradiance data on a tilted
plane.
Concentrating collector
• Parabolic trough technology.
• Tracking and non-tracking case.
• For tracking case use direct-beam irradiance data on
a perfect-tracking plane.
• For non-tracking case use direct-beam on a fixed
plane.
Solar collector data
𝑀tank𝑐p,wd𝑇tankd𝑡
= 𝑞 solar − 𝑞 loss − 𝑞 demand
Capacity = 150 litres.
Dimensions: 1.5 m (height) x 0.36m (ø).
𝑞 solar = 𝑚 coil 𝑐p,w 𝑇coil−in − 𝑇coil−out .
𝑚 coil, 𝑇coil−in
𝑚 coil, 𝑇coil−out
𝑞 demand = 𝑚 coil 𝑐p,w 𝑇supply − 𝑇mains−inlet
𝑞 loss
𝑇room = 20 °C
U-value = 3 W/(m2K).
𝑇coil−out = 𝑇hwc + ∆𝑇pinch .
∆𝑇pinch = 5 °C
𝑞 loss
𝑇tank
Thermal store sub-model
SOLAR THERMAL
COLLECTOR
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
Operational strategies / bypasses
SOLAR THERMAL
COLLECTOR
COLLLECTOR
RECIRCULATE
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
Collector re-circulate
if 𝑇evaporator,out < 𝑇hwc + ∆𝑇pinch
Operational strategies / bypasses
SOLAR THERMAL
COLLECTOR
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
ORC
BYPASS
Collector re-circulate
if 𝑇evaporator,out < 𝑇hwc + ∆𝑇pinch
ORC by-pass:
if 𝑇collector,out > 𝑇ORC,max
Operational strategies / bypasses
SOLAR THERMAL
COLLECTOR
TANK BYPASS
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
Collector re-circulate
if 𝑇evaporator,out < 𝑇hwc + ∆𝑇pinch
ORC by-pass:
if 𝑇collector,out > 𝑇ORC,max
Tank by-pass:
if 𝐹𝑐𝑜𝑖𝑙 < 1
Operational strategies / bypasses
SOLAR THERMAL
COLLECTOR
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
Collector re-circulate
if 𝑇evaporator,out < 𝑇hwc + ∆𝑇pinch
ORC by-pass:
if 𝑇collector,out > 𝑇ORC,max
Tank by-pass:
if 𝐹𝑐𝑜𝑖𝑙 < 1
Rejected heat recovery:
if 𝑚 ℎ𝑤,𝑑𝑒𝑚𝑎𝑛𝑑 > 0
RECLAIM
REJECTED HEAT
Operational strategies / bypasses
SOLAR THERMAL
COLLECTOR
WATER HEATING
CIRCUIT
ORGANIC
RANKINE CYCLE
Collector re-circulate
if 𝑇evaporator,out < 𝑇hwc + ∆𝑇pinch
ORC by-pass:
if 𝑇collector,out > 𝑇ORC,max
Tank by-pass:
if 𝐹𝑐𝑜𝑖𝑙 < 1
Rejected heat recovery:
if 𝑚 ℎ𝑤,𝑑𝑒𝑚𝑎𝑛𝑑 > 0
RECLAIM
REJECTED HEAT
Operational strategies / bypasses
• Cycle evaporation pressure (2 – 30 bar)
• Cycle condensation temperature (15 – 25 °C)
• Working fluid mass flow-rate (0.005 – 0.03 kg/s)
• Collector fluid mass flow-rate (0.005 – 0.07 kg/s)
• % bypass of hot-water cylinder (0 – 100%)
• Type of collector (concentrating/non-concentrating)
• Climate data Irradiance, air temperature
• Demand data Electricity, hot water
Model input variables
• Use model to size components based on required flow rates,
pressures, available roof area etc.
• Market survey to obtain costs for:
solar array
ORC components
domestic hot water cyclinder
ancillary plumbing/installation items
• Calculate the installed cost per unit generating capacity
(£/We) and the Levelised Cost of Electricity (LCoE, £/kWhe).
• Calculate additional hot water production available.
Cost analysis
Rotary screw
compressor
Rotating vane air
compressor
HVAC scroll
compressor
Reciprocating
air compressor
Expander/compressor study
Maximum exergy production from solar collector
.
𝑊 max = 𝐻 sc−out − 𝐻 0 −𝑇0 𝑆 sc−out − 𝑆 0
− d𝑊 = 𝜂Carnot 𝑚 𝑐 ∙ d𝑇sc−out 𝑇0
𝑇sc−out
∆𝐻 = ∆𝑄 sc = 𝜂sc𝐼sol𝐴sc
0 100 200 300 4000
100
200
300
400
Collector outlet temperature [C]
wm
ax [
W]
Parabolic trough
Evacuated tube
Isol = 120 W/m2
Asc = 15m2
Exergy analysis: Reversible vs. endo-reversible
Qh
Qc
W
Th
Tc
Qh
Qc
Th,c
Tc,c
Th,r
Tc,r
W
Maximum power for a 15 m2 solar collector array evaluated over an annual period:
𝑊 max = 192 W when evaluated for reversible Carnot engine
(this is the ideal maximum for this collector).
𝑊 max = 106 W when evaluated for endo-reversible Curzon-Ahlborn engine
(this is a practical expectation for this collector).
Difference in annual exergy production between parabolic trough and evacuated
tube is very small!
Parametric analysis
• Comparison between collector types:
PTC fixed = parabolic tough (concentrating) collector, fixed-orientation.
PTC tracking = parabolic trough collector with ideal 2-axis tracking.
ETC fixed = evacuated tube (non-concentrating) collector, fixed orientation.
• Mass flow rates and operating pressures set for maximum power output from
system for a given collector.
• Maximum power settings for the system affected by the choice of collector
e.g. parabolic trough collector operates at higher temperature hence higher ORC
evaporation pressure.
Parametric analysis – ORC evaporation pressure
0 5 10 15 20 25 300
20
40
60
80
ORC evaporation pressure [bar]
PO
RC [
W(e
)] (
ave
rag
e)
PTC. tracking. msc
= 0.02 kg/s. mwf
= 0.01 kg/s
PTC. fixed. msc
= 0.02 kg/s. mwf
= 0.01 kg/s
ETC. fixed. msc
= 0.03kg/s. mwf
= 0.01 kg/s
Parametric analysis – Solar fluid mass flow rate
0 0.01 0.02 0.03 0.04 0.05 0.060
20
40
60
80
Solar collector fluid mass flow rate [kg/s]
PO
RC [
W(e
)] (a
vera
ge)
PTC, tracking. p2= 18 bar. m
wf= 0.01 kg/s
PTC. fixed. p2= 18 bar. m
wf= 0.01 kg/s
ETC. fixed. p2= 10 bar. m
wf= 0.01 kg/s
Parametric analysis – ORC fluid mass flow rate
0.005 0.01 0.015 0.02 0.025 0.030
20
40
60
80
ORC working fluid mass flow rate [kg/s]
PO
RC [
W(e
)] (a
vera
ge)
PTC, tracking. P2= 18 bar. m
sc= 0.02 kg/s
PTC. fixed. P2= 18 bar. m
sc= 0.02 kg/s
ETC. fixed. P2= 10 bar. m
sc= 0.03 kg/s
Parametric analysis – ORC condensation temperature
15 20 25 30 350
20
40
60
80
ORC condensation temperature [C]
PO
RC [
W(e
)] (a
vera
ge)
PTC, tracking. P2= 18 bar. m
sc= 0.02 kg/s. m
wf= 0.01 kg/s.
PTC. fixed. P2= 18 bar. m
sc= 0.02 kg/s. m
wf= 0.01 kg/s.
ETC. fixed. P2= 10 bar. m
sc= 0.03 kg/s. m
wf= 0.01 kg/s.
Parametric analysis – Hot water provision
0 20 40 60 80 1000
20
40
60
80
Collector fluid flow to hot water cylinder heating coil [%]
PO
RC [
W(e
)] (a
vera
ge)
PTC. tracking.
PTC. fixed.
ETC. fixed.
Performance analysis - 24 hour period
4 6 8 10 12 14 16 18 200
100
200
300
400
500
600
Time [hour]
PO
RC [
W(e
)]
PTC. tracking. P2= 18 bar. m
sc= 0.02 kg/s. m
wf= 0.01 kg/s
ETC. fixed. P2= 10 bar. m
sc= 0.03 kg/s. m
wf= 0.01 kg/s
Intermittent operation observed for a fixed flow rate system:
Results of annual simulation with fixed flow rates
PTC tracking
PTC fixed
ETC fixed
Average electrical output We (avg) 75 44 67
Installed cost per We £/We (avg) 61.5 103.8 37.1
Peak electrical output We (peak) 386 384 321
Total annual electrical output kWeh/yr 657 389 588
% annual electrical demand % 19.9 11.8 17.8
ORC switch-on temperature °C 137 137 105
ORC operation time hr/yr 1836 1090 2080
Cooling water consumption m3/yr 834 502 956
Avg solar collector efficiency* % 46.5 31.3 51.5
Avg ORC efficiency % 14.2 14.2 11.9
Initial investment cost £ 4614 4615 2489
Annual incurred (O&M) cost £/yr 46 46 25
Levelised cost of electricity £/kWh 0.80 1.36 0.49
Payback time years 13.9 14.8 9.3
Annual CO2 emission savings kgCO2/yr 391 251 355
*Reported value is the mean solar collector efficiency during ORC operational hours only, and
normalised relative to the global (diffuse + direct) solar irradiance on a horizontal surface.
Summary and conclusions
• Assessment of a small-scale combined solar heat and power system based on
organic Rankine cycle technology for domestic use in the UK.
• System model based on simple component efficiency data, load profiles and
operational control regimes.
• Annual simulation results for a fixed flow-rate system with a 15 m2 solar collector
array show an average power output in the region 65-75 We(avg).
• Installed cost of system in the region £2500-4500.
• Cost per unit power 37-62 £/We(avg) compared to 20-30 £/We(avg) for solar-PV.
• Potential to achieve a further ~30% increase in power output from the system
through optimisation of ORC design and control of system flow rates.
Planned future developments
• Control of system including variable flow rates
• Options for heat rejection
• Consideration of most appropriate working fluid or mixture
• Consideration of appropriate expanders for small-scale ORC
• Further consideration of solar collector design
• Experimental validation of solar collector and ORC sub-models
• Combined levelised-cost comparison with PV and Hybrid PV-T
Next step: variable flow rate control
• Prevent periodic on-off switching of system by not extracting too
much heat from the collector fluid.
• Modify 𝑚 wf so that 𝑄 ORC−in ≤ 𝑄 collector−in.
• 8% increase in annual work production achieved.
6 8 10 12 14 16 180
50
100
150
200
250
300
350
Time [hour]
Pe
l [W
]
Fixed WF flow rate, 0.01 kg/s
Variable WF flow rate
4 6 8 10 12 14 16 18 200
20
40
60
80
100
120
140
160
Time [hour]T
sc-o
ut [ C
]
Fixed WF flow rate, 0.01 kg/s
Variable WF flow rate
Thank you.
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power plant”, in DOE Solar Energy Technologies Program Review Meeting, (Denver, USA), 2004.
Department for Energy and Climate Change (DECC). 2012. Personal communication with authors.
P. Owen, “Powering the Nation. Household electricity-using habits revealed.” Energy Saving Trust, 2012.
S. Quoilin, M. Orosz, H. Hemond, and V. Lemort, “Performance and design optimization of a low-cost solar
organic Rankine cycle for remote power generation”, Solar Energy, vol. 85, no. 5, pp. 955-966, 2011.
Office of Gas and Electricity Markets, “Typical domestic energy consumption gures factsheet." [Online], 2011.
Available: http://www.ofgem.gov.uk
Solartechnik Prϋfung Forschung, “Collector catalogue." CD, 2002. Produced by Institut fr Solartechnik SPF,
Rapperswil, Berne.
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Cooling Programme, 2008.
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