Low-Temperature Solar Rankine Cycle System for Reverse Osmosis Desalination Project coordinator Agricultural University of Athens Dept. of Agricultural Engineering, Farm Structures Laboratory Cooperative Research project Contract No.: COOP-CT-2003-507997 Contact person: Mr. Dimitris Manolakos, Mechanical Engineer
1
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Solar Energy and Desalination Mechanical work
Electricity PV Solar Energy
Rankine engine Collectors
Membranes Product (fresh water)
Evaporation Heat
2
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
System Layout 1. High efficiency vacuum tube solar collectors’ array 2. Circulator 3. Preheater-Evaporator 4. Condenser 5. Expanders 6. HFC-134a pump 7. RO unit 8. Insulated seawater reservoir 9. Fresh water reservoir 10. RO energy recovery system
3
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
The Rankine cycle
4
4’
3
1
2
1→2: Isentropic expansion (expander) 2→3: Isobaric heat rejection (condenser) 3→4: Isentropic compression (HFC-134a pump) 4→1: Isobaric heat supply (evaporator)
4
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Thermodynamic states
5
State
T (oC)
P (kPa)
H (kJ/kg)
S (kJ/kgoK)
S1 Super-heated vapour, evaporator outlet, expander inlet
75.8
2200
435.7
1.7138
S2 Saturated vapour, condenser inlet
35
887.91
417.5
1.7138
S3 Saturated liquid, condenser outlet
35
887.91
249.2
1.1676
S4 Sub-cooled liquid, pre-heater inlet
≈35
2200
248.0
1.1676
S4’ Saturated liquid, evaporator inlet
71.7
2200
307.8
1.3433
expander
outlet
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Why this System? (1) z
Use of market available components (heating and cooling industry, at low cost) Ideal exploitation of low temperature energy sources
z
Mechanical work is driven directly to RO pumps Æ direct efficiency gain
z
It can be easily standardised
z
Rankine cycle approaches the efficiency of Carnot cycle
z
Continuous and safe operation at low temperatures.
z
The working fluid (HFC-134a) is not corrosive and is environmentally friendly.
z
Low maintenance cost
z
Continued…
6
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Why this System? (2) zIt
fits perfectly for applications in isolated, not grid connected areas.
zCompared
to PV-RO desalination system this system prevails in the following:
–
Water storage is used instead of batteries
–
It is more environmentally friendly
–
The absence of batteries implies less maintenance
–
The system is safer for the end users.
–
No qualified staff is needed for O&M.
–
The fresh water cost is expected to be at competitive level.
to thermal systems is characterised by a much higher efficiency and much less product water cost. zCompared
pressure working conditions in RO system Æ higher energy availability and higher efficiencies in the whole system zVariable
7
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Collectors
Components size
Manufacturer
Thermomax Ltd.
Type
SOLAMAX
No. of tubes/collector
30
No. of collectors
56
No. of collectors connected 2 in series Slope (o)
8
40
Preheater
40 kW
Evaporator
65 kW
Condenser
100 kW
RO Unit
1 m3/h
Expander
Scroll type
Freon pump
Piston/diaphragm 0.9 kW
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Energy balance of the system
9
Collectors’ energy gain (MWh/y)
101.3
HFC-134a pump (MWh/y)
0.89
Energy for condensation (MWh/y)
90.83
Energy for preheating (MWh/y)
35.5
Energy for evaporation (MWh/y)
65.8
Energy from expanders (MWh/y)
7.10
System efficiency (%)
7
Energy for desalination (MWh/y)
2.53
Fresh water produced (m3/y)
1012
Specific energy consumption (kWh/m3)
2.5
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
The collectors’ field
10
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Collectors’ array
11
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Rankine engine
12
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Cost analysis Building (10m²) Land Rent (0.1 hectare)
Total cost (€)
% of total
Condenser (100kW)
5500
2.87%
5000
2.61%
Freon pump (0.9kW)
7600
6.40%
Pipes
1500
0.78%
Expanders (7kW)
1236
0.64%
Rankine labour
4000
2.09%
300
Energy System
50.59%
Collectors part
35.00%
Solar collectors (100.1kW)
50400
Collectors Pipes and installation
25000
Collectors pump
440
Rankine cycle part
13
1.95%
23.24% 11.53% 0.23% 15.59%
Preheater (40kW)
2700
1.41%
Evaporator (65kW)
2700
1.41%
AGRICULTURAL UNIVERSITY OF ATHENS
32.37%
Desalination System (RO unit 1m³/h) Membranes
4000
7.33%
All other components
40000
25.04%
8000
4.17%
(Labour included) Civil works
8.31%
Others Instrumentation
5000
3.13%
Tanks
800
0.42%
26/09/2005
Cost sharing Civil w orks 4% Desalination System 32%
Building (10m²) 3% Others 8%
Land Rent (0.1 hectare) 2%
With collectors
Energy System 51%
Others 15% Civil w orks 2%
With other thermal source
14
AGRICULTURAL UNIVERSITY OF ATHENS
Building (10m²) 4%
Land Rent (0.1 hectare) 3% Energy System 25%
Desalination System 51%
26/09/2005
Cost by quantity of fresh w ater produced 50,0
160
45,0
140
40,0
€/m³
Cost of w ater produced per month (€/m³)
120
35,0 30,0
100
25,0
80
20,0
60
15,0
40
10,0 5,0
20
0,0
0 1
2
3
4
5
6
7
8
m³
Fresh w ater produced per month (m³)
9 10 11 12
Months
15
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Conclusions zA
7% overall system efficiency is expected. It can be higher if operates at higher temperatures.
zAlternative
to PV-RO systems, BUT with less O&M cost, longer life time at not much higher water price
zIdeal
exploitation of low temperature energy sources like thermal wastes, geothermal energy for fresh water production.
zAverage
fresh water cost 15.21 EUR/m3
BUT In case it is supplied from a steady thermal source (e.g. geothermal, co-generation plant etc.) the cost is reduced to 1.18 EUR/m3
16
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
Acknowledgements The Project is realised under the COOP-CT-2003-507997, partially financed by European Commission.
17
AGRICULTURAL UNIVERSITY OF ATHENS
26/09/2005
