cost and energy efficiency optimization of...
TRANSCRIPT
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Cost and energy efficiency optimization of vapor compression
systems
Gunda Mader
CopenhagenDept. of Mechanical EngineeringEnergy Engineering Section
Nordborg, DKStockholm
Dept. of Energy Technology
Applied Thermodynamics and Refrigeration
DTUDanfoss A/SKTH
October 2011 - Slide 2
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» Motivation
» Goals
» Focus application
» Approach
» Objective functions» Energy efficiency
» Cost related evaluation criteria
» Cycle screening» “Thermodynamic approach”
» “Economic approach”
» Summary & Outlook
Content
October 2011 - Slide 3
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» The strong demand to » increase energy efficiency, » reduce costs and » reduce the use of environmentally hazardous refrigerants
changes the game of designing vapor compression systems.
» Various technologies like» variable speed compressors» electronic expansion valves» microchannel heat exchangers
start to be attractive for small residential applications.
» This development dramatically increases the degrees of freedom for finding a competitive system design regarding both» component design and» system layout.
The Motivation to do research for a technology which is
more than 100 years old comes from recent changes :
October 2011 - Slide 4
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» Developing a generic top-down approach to » systematically analyze a heat pumping or refrigeration task,
» filter out unfeasible solution approaches and » find cost and energy efficiency optimal system, subsystem and component solutions.
» Analyzing for the focus application residential heat pump systems the influence of reduced charge on cost and energy optimal solutions.
» Thereby accelerating the development and introduction of new solutions that enable a charge reduction.
Hence the Goal of the project is speeding up the process of
determining the optimal design of a vapor compression system by:
October 2011 - Slide 5
syseff» Air/water heat pump
» space heating (floor, radiator, retrofit)
» capacity: single/double family houses
The actual work of developing the method is done for a
Focus application:
*3rd EHPA European Heat Pump Forum, Brussels 2010, Forsén**VDI Tagung 2010, Miara
Brine/water versus Air/water heat pumps in Europe*
2005
26%
74%
air/water
brine/water
2009
64%
36%
Brine versus air inlet temperature (Germany, averaged)**
2005
2009
air/waterbrine/water 0
-10
10
20
air brine
The choice is motivated by the market development and the challenge of operating under largely varying conditions:
October 2011 - Slide 6
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» Cycles
An analysis of the Focus application results in
a decision space. A heat pump can be designed by combining each of the different options:
16 Staged cascade14 Ejector13
Separator + IC +
SLHX
15 Standard cascadePhase separator
11Closed Economizer
full subcooling
5 6
Closed Economizer
partial subcooling
Oil cooler
12Economizer + IC +
SLHX9 10
Parallel
compression
Intercooler 7 8 Open EconomizerIC + SLHX
1 Baseline 2 SLHX 3 4 Expander
Ev
C
Ev
Co
Eva
Co
Eva
Con
Ev
Co
Eva
Co
Eva
Con
Ev
C
Ev
Co
Ev
Co
C
Ev
Ev
Co
R717
E170
R152a
R32
R134a
R1270
R290
R407C
R410A
R423A
R143a
R404A
R507A
R125
R218
R1234yz
» Refrigerants» Components
» Control
October 2011 - Slide 7
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» Superstructure for multi-objective optimization» mixed integer non linear programming
» e.g. evolutionary algorithm
» But:» modeling challenge
» problems of numerical stability
» result evaluation complicated (black box)
» infeasible computation time
An obvious Approach would be to create a
October 2011 - Slide 8
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» Characterization of the taskExtrinsic characteristicsIntrinsic characteristicsDecision space
» Refrigerant screening
» Cycle screeningBasic thermodynamic cycle computation Mapping cost index over computed efficiency
» Optimization of component selectionTrade-off between system versuscomponent costs for optimized component selection.
An alternative Approach separating the optimization
problem into different steps is developed here. By deselecting infeasible solutions in each step the mentioned challenges but also problems of limited availability of information should be tackled
How to measure energy efficiency?How to quantify cost?
Energy efficiency
Cost
Solution 1Solution 2
Solution 3
Energy efficiency
Cost
1
2
3
Solutions:
Energy efficiency
Cost
Solution 1Solution 2
Solution 3
Energy efficiency
Cost
1
2
3
Solutions:
Energy efficiency
Cost
Solution 1Solution 2
Solution 3
Energy efficiency
Cost Cycles:
3
2
1
Energy efficiency
Cost
Solution 1Solution 2
Solution 3
Energy efficiency
Cost Cycles:
3
2
1
October 2011 - Slide 9
syseffQ [kW]
» Seasonal coefficient of performance (SCOP)
» Standard prEN 14825
» defines load, air & water temperatures, water flow rates
The Objective functions must be defined
0
2
4
6
8
10
12
14
16
18
20
-25 -15 -5 5 15
0
100
200
300
400
500
600
Tamb [oC]
Ql Qd
heating hours [hrs/yr]
Tbi
Ql
Qd
Tbi
… load
… delivered (fixed speed)
… bivalenttemperature
Energy efficiency
A climate profile is defined by the heating hours per year. The demand (load) of the building depends linearly on the air temperature.
October 2011 - Slide 10
syseffThe Objective functions must be defined:
Cost related evaluation criteria
“Component costs”with an objective means of measurement
»component sizes: HX area, compressor volume, air flow rate, others -> from model
“Cycle complexity”with no (obvious) objective means of measurement
»part load capability»ease of control»ease of oil management»ease of cycle reversibility & defrost»component technology readiness»cycle technology knowhow»flammability/toxicity management
HOW SHOULD THESE DIFFERENT COST RELATED CRITERIA BE COMPARED?
October 2011 - Slide 11
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A) relate all criteria to costs» ‘what does it cost to increase compressor discharge volume’» ‘what does it cost to develop a controller’
B) weigh all criteria by evaluating preferences (evaluation based on knowledge and intuition)» ‘increasing HX area by dA is strongly preferred to increasing
control complexity’» ‘increasing air flow rate is weakly preferred to …’
C) separate component costs and system complexity
The Objective functions must be defined:transfer problem into a
single criterion problem
Cost related evaluation criteria
But only limited information is available for all possible cycles!
But this requires to compare too different things, hence the questions can’t be answered reliably!
October 2011 - Slide 12
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How can cycles be compared in the Cycle screeningwithout a detailed definition of the components? Thermodynamic approach:
» compare for components with equal efficiency
» But:
» component sizes (=costs) vary for different cycles & refrigerants
» efficiency varies with operating condition
for one operating condition only!
heat exchanger efficiency heat exchanger efficiency
different results for different refrigerants (+ cycles)
total heat exchanger size
equal hx efficiencies require different component sizes for different refrigerants (+ cycles)
October 2011 - Slide 13
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How can cycles be compared in the Cycle screeningwithout a detailed definition of the components? Economic approach:
» compare for components with equal size (= cost)
SCOP
cycle
complexity Refrigerant 1
Refrigerant 2
Cycle 1
Cycle 2
» But:
» how to choose lower/upper size for components?
» lower/upper component size = min/max SCOP for the size range!
smallest componentsizes
biggest component
sizes
October 2011 - Slide 14
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SCOP
cycle
complexity
How can cycles be compared in the Cycle screeningwithout a detailed definition of the components? Economic approach:
UAe
UAc
Cair
Using statistical methods the simulation effort to develop a quadratic regression modelcovering the “space” of all component sizes can be minimized:
upper/lower boundary represent min/max SCOP for the given component size range
compare points of equal component size
October 2011 - Slide 15
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How can cycles be compared in the Cycle screeningwithout a detailed definition of the components? Economic approach:
» method tested so far for varying evaporator size, condenser size, air mass flow rate» numerical effort reasonable
» quadratic models in good agreement with simulation models
» Outlook: investigation to include» compressor discharge volume
» operating constraints
» optimization of control parameters
» other intrinsic characteristics
October 2011 - Slide 16
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» Cycle screening by comparing cycles at equal component size
» Optimization of component
selection:
» Optimization under limited availability of information
» Optimization problem separated in two parts» cycle screening: optimization of refrigerant/cycle layout
» optimization of component selection
Summary & Outlook