Performance testing of solar hot waters stores Research and Test Centre for Thermal Solar Sysems (TZS) Universität Stuttgart, Germany Pfaffenwaldring 6, 70550 Stuttgart Email: drueck@itw.uni-stuttgart.de Internet: www.itw.uni-stuttgart.de 1 File: STO_TEST_HD1.PPT
Integration of hot water store in solar thermal system Source: ELSE07\STO_SYS1.CDR 2
Thermal Testing of stores - why? Selection of appropriate store comparison and assessment of stores component based test methods (z. B. ENV 12977-2) 3
But: Testing is not enough! ---> Important is the behaviour of the store within the system usually determined by means of annual system simulations e. g. with TRNSYS, T*Sol or Polysun ---> numerical model of store required 4
Two types of numerical models real physical models parametric models ρ + t ( ρ w ) ( ρ w ) ( ρ w ) x x + y y + z r w r r r ρ + η t ρu t 2 ( w ) w = k p + w r z r = 0 En + ρwu Φ En = * ( λ T ) + q& + Diss ϑ i,j-1 ϑ. ini,j Q ini,j ϑ i+1,j. Q i+1,j.. Q i,j-1 ϑ i,j. Q i-1,j ϑ i-1,j Q i,j+1 ϑ ambi,j. Q ambi,j ϑ i,j+1 5
Requirements on numerical models The thermal behaviour of the store must be described with a sufficient accuracy short computation times ---> one-dimensional models (parametric) What does this mean for 1D models 6
The consequence for 1D - models The models need not to describe the real physical phenomena inside the store but the thermal behaviour of the store Description of the result of complex heat and flow processes that take place inside the store by means of one-dimensional models 7
How to test a hot water store? test method for hot water stores numerical store model + test sequences + PI-algorithm = store parameters 8
Parameters required for description of thermal behaviour thermal capacity heat loss rate heat transfer capacity rate of solar loop heat exchanger heat transfer capacity rate of auxiliary loop heat exchanger auxiliary volume thermal stratification during discharge thermal stratification during stand-by 9
Heat losses of store --> heat losss capacity rate (UA) S,a Quelle: Folien95\Folie1.CDR 10
Characterisation of heat exchangers --> heat transfer capacity rate (UA) hx 11
Characterisation of heat exchangers But: heat transfer capacity rate is not constant 12
Degradation of thermal stratification during stand-by modelling by means of effective thermal conductivity λ eff 13
Degradation of thermal stratification during direct discharge --> modelling by a stratification number 14
Parameters required for description of thermal behaviour thermal capacity heat loss rate heat transfer capacity rate of solar loop heat exchanger heat transfer capacity rate of auxiliary loop heat exchanger auxiliary volume thermal stratification during discharge thermal stratification during stand-by 15
How to determine all this parameters? test method for hot water stores numerical store model + test sequences <--- + PI-algorithm = store parameters 16
Basic requirement for store test method no measurements inside store ----> Black-Box approach 17
Example for test sequences of hot water store test sequences designed in a way, that individual parameters are stimulated testsequence C S L NA NB aim heat capacity of entire store heat transfer capacity rate of solar loop heat exchanger degradation of thermal stratification during direct discharge heat loss rate (during stand-by) heat capacity auxiliary volume heat transfer capacity rate of auxiliary loop heat exchanger degradation of thermal stratification during stand-by operation with solar loop heat exchanger operation with axuiliary loop heat exchanger 18
How to determine all this parameters? test method for hot water stores numerical store model + test sequences + PI-algorithm <--- = store parameters 19
Principle of parameter identification 20
How to determine all this parameters? test method for hot water stores numerical store model + test sequences + PI-algorithm = store parameters 21
Example for the application of the method on a typical solar hot water store Parameters to be determined thermal capacity heat loss rate heat transfer capacity rate of solar loop heat exchanger heat transfer capacity rate of auxiliary loop heat exchanger auxiliary volume thermal stratification during discharge thermal stratification during stand-by 22
comparison of results from three tests store parameters Test 1 Test 2 Test 3 useful store volume V s [Liter] heat loss rate (UA) S,a [W/K] auxiliary volume V aux [Liter] stratificaiton number N [-] effective vertical thermal conductivity λ eff [W/(mK)] 266 266 269 1,95 2,08 2,11 130 133 132 112 119 105 1,81 1,63 1,57 temperature dependet heat transfer capacity rate solar heat exchanger K WT1 [W/K] b WT1,3 [-] 49,4 0,567 37,7 0,642 47,1 0,591 temperature dependent heat transfer capacity rate auxiliary hx K WT2 [W/K] b WT2,3 [-] 33,2 0,594 55,8 0,471 64,3 0,436 23
comparison of results from three tests heat transfer capacity rate of solar loop heat exchanger Wärmeübertragungsvermögen (UA) WT1 [W/K] 700 600 500 400 300 200 100 0 (UA) WT1 = K WT1 ϑ m b WT1,3 Test 1 Test 2 Test 3 20 30 40 50 60 70 80 mean mittlere local lokale temperature Temperatur ϑ m [ C] 24
comparison of results from three tests heat transfer capacity rate of auxiliary loop heat exchanger Wärmeübertragungsvermögen (UA) WT2 [W/K] 500 400 300 200 100 0 (UA) WT2 = K WT2 ϑ m b WT2,3 Test 1 Test 2 Test 3 20 30 40 50 60 70 80 mean mittlere local lokale temperature Temperatur ϑ m [ C] 25
comparison of results from three tests store parameters Test 1 Test 2 Test 3 useful store volume V s [Liter] heat loss rate (UA) S,a [W/K] auxiliary volume V aux [Liter] stratificaiton number N [-] effective vertical thermal conductivity λ eff [W/(mK)] 266 266 269 1,95 2,08 2,11 130 133 132 112 119 105 1,81 1,63 1,57 temperature dependet heat transfer capacity rate solar heat exchanger K WT1 [W/K] b WT1,3 [-] 49,4 0,567 37,7 0,642 47,1 0,591 temperature dependent heat transfer capacity rate auxiliary hx K WT2 [W/K] b WT2,3 [-] 33,2 0,594 55,8 0,471 64,3 0,436 fractional energy savings f sav [%] 56,6 56,0 56,1 26
Conclusions and future perspectives The presented method delivers reproducible results --> method standardised in ENV 12977-3 --> extension of the method to combistores (stores of solar thermal systems for combined domestic hot water preparation and space heating) 27