Studies of Control Strategies for Building Integrated Solar Energy System

Transcription

1 Studes of Control Strateges for Buldng Integrated Solar Energy System anan Abd Wahab a,c, Mke Duke a, James K. Carson a, Tm,Anderson b a School of Scence and Engneerng, Unversty of Wakato,Prvate Bag 305, amlton 3240 New Zealand b School of Engneerng Auckland Unversty of Technology, Prvate Bag 92006, Auckland 42, New Zealand c Unverst Tun ussen Onn Malaysa, Beg Berkunc 0 Part Raja,Batu Pahat, 86400, Johor, Malaysa Abstract Research and development work on Buldng Integrated Solar Energy Systems (BISES) has become an area of growng nterest, not only n New Zealand (NZ) but worldwde. Ths nterest has led to a sgnfcant growth n the use of solar energy to provde heatng and electrcty generaton. Ths paper presents the theoretcal and expermental results of a novel buldng ntegrated solar hot water system developed usng commercal long run roofng materals. Ths work shows that t s possble to acheve effectve ntegraton that mantans the aesthetcs of the buldng and also provdes useful thermal energy. The results of a 6.73m 2 glazed domestc hot water systems are presented. The key desgn parameters of the Buldng Integrated Thermal (BIT) system were dentfed and mplemented n a TRansent SYstem Smulaton (TRNSYS) model. Valdaton results comparng the smulaton n TRNSYS and real expermentaton show that expermental and smulaton responses are close to each other. The couplng of TRNSYS and Matlab/Smulnk shows the possblty to use Matlab/Smulnk for developng approprate control strateges for BIT roofng systems. Prelmnary Fuzzy ogc (F) ntellgent controller was mplemented n a Fuzzy Integrated System (FIS) toolbox n a Matlab/Smulnk model and lnked nto TRNSYS model. Further work s needed to dentfy and desgn advanced predctve control strateges for the Buldng Integrated Photovoltac Thermal (BIPVT) solar system and determne how the performance can be optmzed. Keywords-BIT;BIPVT;TRNSYS;Matlab/Smulnk I. BACKGROUND Tradtonally, research n the feld of solar water heatng has been conducted n relatve solaton from the buldng ndustry. Although ths has led to the development of hgh performance systems, there appears to have been lttle consderaton nto the ntegraton of these systems wth the buldngs they are typcally used n. In a study by Probst et. al [] t was shown that there are a number of factors that need to be addressed n order to acheve better ntegraton of solar devces and the bult envronment. In partcular they note that future buldng ntegrated solar collectors should be conceved as part of a constructon system. Ths vew was also expressed by PV Catapult project [2] when reflectng on the ntegraton of photovoltac systems nto buldngs. The use of water heatng solar collectors as buldng elements has, untl recently, been largely gnored. J et al. [3] and Chow et al. [4] both examned a photovoltac/thermal system for ntegraton nto buldng walls n ong Kong. They showed that these systems could make useful heat gans whle also actng to reduce thermal load on the buldng. owever these systems were essentally ntegrated onto a buldng rather than nto the buldng (.e. ndvdual collectors were used as the materal for the wall, rather than usng the wall as the materal for a collector). Smlarly, Kang et al. [5] dscussed the performance of a roof ntegrated solar collector whch agan conssted of a seres of standalone collectors used as a roof. Accordng to Probst et. al [] ths method of ntegratng solar collectors s consdered to be acceptable to archtects, but s stll only demonstratng the ntegraton of collectors onto a buldng rather than nto the buldng. Medved et al. [6] however examned an unglazed solar thermal system that could be truly ntegrated nto a buldng. In ther system they utlsed a standard metal roofng system as a solar collector for water heatng. They found that n a swmmng pool heatng system, that they were able to acheve payback perods of less than 2 years. Ths translated to a reducton of 75% n the tme taken to pay for a glazed solar collector system. Smlar systems to that of Medved et al. have been developed and dscussed by Bartelsen et al.[7], Colon and Merrgan [8] and Anderson et. al [9]. owever, despte the recent research and the recognton of the market for buldng ntegraton of solar collectors, the work

2 undertaken n the feld s relatvely small n comparson to work on stand-alone collectors. Although standalone collectors can successfully be ntegrated onto buldngs t has been suggested that ths does not necessarly result n an attractve fnsh. As a result, ths study ams to examne the performance of a buldng ntegrated solar thermal collector based on sheet metal roofng that dsplays a greater level of ntegraton, and satsfes more of the requrements dentfed n the lterature than many of the prevous systems. II. BUIDING INTEGRATED TERMA SYSTEM In NZ and Australa long run metal roofng s wdely used for domestc, commercal and ndustral applcatons. A typcal example of such a roof s shown n Fg.. Fg. 2: Schematc of BIT Panel A prevous theoretcal study and small scale testng of Buldng Integrated Photovoltac Thermal (BIPVT) panels [9] had been undertaken by the Solar Engneerng Research Group at the Unversty of Wakato. BIPVT s a combned system that generates both electrcty and hot water. The panels are dentcal to the BIT panels but have photovoltac cells lamnated onto the collector plate. The thermal performance of optmsed BIPVT compared to commercally avalable flat plate solar thermal collectors s shown n Fg. 3. Fg. : ong Run Metal Roof ong run roofng comprses a substrate of steel strp, commonly 0.40 mm or 0.55 mm thck and coated wth 45% znc, 55% alumnum alloy. A corroson nhbtve prmer and top coat (pant) are appled to the outer surface and s avalable n a wde varety of colours. The fnshed sheet s then roll formed or folded nto the desred profle. An nvestgaton was undertaken to determne f commercally avalable panted steel was sutable for use drectly as a buldng ntegrated solar thermal (BIT) panel. Two metre lengths of black panted steel were manufactured usng a CNC foldng machne. Durng the foldng process a flud channel, 35 mm wde was ncorporated. Manfolds and end plugs were added. Fnally a black panted steel collector plate was glued over the flud trough as shown n Fg. 2. The collector plate absorbs solar energy. As water or heat transfer flud flows up the channel, heat s transferred from the undersde of the collector plate to the flud. Prevous research [9] showed that steel s an effectve materal for a buldng ntegrated solar collector plate f the channel wdth s hgh, typcally more than 20mm. Therm al Effcency (T -T a )/G" (m 2 K/W) gh performance glazed flat plate (SPF, 2007) Expermental glazed flat plate Glazed BIPVT Unglazed BIPVT Fg. 3: Theoretcal and Expermental Performances of Optmsed BIPVT Collectors It can be seen that the effcency of the optmsed glazed BIPVT s lower but stll good enough to provde useful thermal energy n sunny regons such as Australa and relatvely sunny regons such as NZ. owever, n ths paper no photovoltac cells were ncluded so that the expermental rg operated as BIT only. One of the ams of the experments was to determne how purely BIT performance compared to BIPVT. A basc schematc dagram of the BIT system s shown n Fg. 4.

3 III. TESTING AND RESUTS Fg. 4: Schematc dagram of the BIT system To nvestgate the performance of glazed BIT, a solar water heatng system was bult usng a smlar constructon method to a conventonal long run metal roof (see Fg. 5). A. Performance Testng Performance testng of the glazed black BIT panels was undertaken to determne ther effcency when n a real nstallaton and to nvestgate the maxmum water temperatures possble. To acheve ths, a small nsulated tank was flled wth ~35 ltres of water at ambent temperature. On a clear sunny day, wth average solar nsolaton of 929 W/m 2, the pump was swtched on and the water crculated through the glazed BIT. The nlet and outlet temperatures were measured along wth the flow rate and solar nsolaton. The system operated all day and nght. Nght tme runnng allowed the water to be cooled by radaton ready for the next day s testng. The water temperature for a good summer s day s shown n Fg. 6. It can be seen that the maxmum temperature reached was approxmately 90 C. Ths s well above the requred C of domestc hot water and demonstrates that glazed BIT can reach the requred temperature. Tem p,c :00:30 :00:30 2:00:30 3:00:30 4:00:30 5:00:30 6:00:30 7:00:30 8:00:30 9:00:30 0:00:30 :00:30 2:00:30 3:00:30 4:00:30 5:00:30 6:00:30 7:00:30 8:0:30 9:0:30 20:0:30 2:0:30 22:0:30 23:0:30 Fg. 6: Water Temperature from Collector Fg. 5: Glazed BIT Test Rg The BIT was nstalled usng standard buldng paper, rafters, battens and nsulaton. Folded polycarbonate sheets were used for the glazng on the black BIT panels. The test rg enabled the performance of glazed BIT to be evaluated almost as f t had been nstalled on an actual buldng. The rg comprsed three parallel rows of eght coloured BIT panels n seres, black, green and grey. Each row was plumbed so they could operate ndependently of the others, allowng for comparatve testng of collectors of dfferent colours. Intal tests showed a flow dstrbuton problem wth eght panels n seres. The central panels had lttle or no flow so the panels were splt nto groups of four n seres. Ths resolved the problem but hghlghted a potental problem wth the manfolds. The thermal effcency (η) can be determned drectly from the expermental results based on the ottel-whller equaton [0]. It s defned smply as the rato of heat transfer n the collector Eq. to the product of the collector area and the global solar rradance, as shown n Eq. 2. & () Q = mc & p T Q& = A G" η (2) collector From the expermental data, the effcency of a solar collector for all condtons can be represented by a lnear equaton of the form shown n Eq. 3. T Ta η = η0 A a (3) G"

4 Where: G " = solar rradance (W/m 2 ) m& = mass flow rate (kg/s) C p = specfc heat of the collector coolng medum (J/kg/ o C) T = dfferences between flud out temperature, T o and nlet temperature T A collector = collector area (m 2 ) T = nlet temperature ( o C) T = ambent temperature ( o C) a BIT stll performed well enough to be an effectve solar hot water heater system. B. Model Valdaton The valdaton process s done by comparng between real experment and smulaton responses. The key desgn parameters of the BIT system were dentfed and mplemented n a TRNSYS model. Fg. 8 shows the expermental and smulaton result for outlet temperature of the BIT collectors. The expermental and smulaton responses are close to each other. Therefore, t can be concluded that the BIT system modeled n TRNSYS s good enough to represent the actual BIT system. η OA = collector optcal effcency Based on the expermental data t was possble to derve the effcency equaton for BIT collector analyss usng a lnear least square regresson analyss [9].The result from the expermental data measured durng testng s shown n Eq.4. T T = G" a η (4) The sgnfcance of the effcency equatons can be better understood from an nspecton of Fg. 7. T e m p e r a t u r e ( C ) Tme (hour) Smulaton Expermental Fg. 8: Expermental and Smulaton Response Therm al Effcency y = x Theory Expermental near (Theory) near (Expermental) y = x (T-Ta)/G" Fg. 7: Theoretcal and Expermental Performance of BIT system When tryng to realse a practcal BIT system compromses had to be made to acheve a real world.consequently the collector surface, optcal propertes of the glazng and the fn effcency were not as good as an optmsed glazed BIPVT [9] resultng n a lower thermal effcency. None the less the glazed IV. ATERNATIVE CONTRO STRATEGIES FOR IMPROVING SYSTEM PERFORMANCE Effcent control strateges are essental for the effectve operaton of a solar energy system, and must be consdered a fundamental part of the desgn []. In ths research, the control strateges were desgned and smulated n Matlab/Smulnk envronment, whle the performance of the solar energy system were calculated wth a TRNSYS smulaton model. The couplng of Matlab and TRNSYS leads to a powerful tool that enables the user to combne the advantages of each program: the modern modelng and solvng technques of Matlab as well as exstng and proven, well-valdated models and utlty routnes of TRNSYS. For ths study the standard TRNSYS solar domestc hot water (SDW) system dagram was modfed for BIT system model as shown n Fg. 9. The BIT system nputs, varables and parameters of each component (pump, tank, solar radaton data, collector, etc) were defned, from the expermental data, n the TRNSYS model to perform the smulaton. The Type 55 component was used to mplement a lnk wth Matlab. Ths allows any Matlab command (ncludng Smulnk smulatons) to be run wthn a TRNSYS smulaton.

5 Fg.0:Smulnk Model Fg. 9: BIT System Model n TRNSYS In the frst nstance a standard On/Off controller was desgned usng Smulnk tool n the Matlab envronment. The mathematcal equatons of the On/Off control functon are expressed as follows: Both the smulated response for standard On/Off control n TRNSYS and the On/Off control functon desgned n Matlab/Smulnk are shown n Fg.. The results showed good agreement. Outlet Temp-TRNSYS Inlet Temp-TRNSYS Outlet Temp-Smulnk Inlet Temp-Smulnk Flowrate-TRNSYS Flowrate-Smulnk If the controller was prevously ON f = γ and ( T T ), γ = 0 f = T (5) γ and > ( T T ), γ = 0 T (6) If the controller was prevously OFF f = 0 γ and ( T T ), γ = 0 f = 0 T (7) γ and > ( T T ), γ = 0 Where: T (8) T e m p e r a t u r e ( C ) Tme (our) F l o w r a t e ( ( k g / h o u r ) T T T T γ [0..] γ 0[0..] = upper dead band temperature dfference = lower dead band temperature dfference = upper Input temperature = lower Input temperature = nput control functon = output control functon Fg. : Comparson Smulated Result Now BIPVT s a combned system that generates both electrcty and hot water. The panels are dentcal to the BIT panels but have photovoltac cells lamnated onto the collector plate. Fg. 2 shows a BIPVT system n TRNSYS model whch had been undertaken n prevous study [9]. The On/Off control functon was desgned usng the Smulnk tools n Matlab. Smulnk provdes a graphcal user nterface (GUI) to assst n desgnng control systems. Fg. 0 shows the Smulnk model for On/Off control functon.

6 V. CONCUSIONS The BIT solar collectors performed well and reached the requred temperature for domestc hot water systems. The thermal performance of a solar roofng system was evaluated numercally and expermentally. The expermental effcency s n good agreement wth the theoretcal result. Valdaton results n TRNSYS comparng the smulaton and real expermentaton show that expermental and smulaton responses are close to each other. Couplng TRNSYS and Matlab/Smulnk shows the possblty to use Matlab/Smulnk for predctve control strateges for BIPVT system. Further work s needed to dentfy and desgn an advanced predctve control strateges for BIPVT system and determne how the performance can be optmzed. Fg. 2: BIPVT System Model n TRNSYS [9] A Fuzzy ogc (F) ntellgent controller for BIPVT system was desgned n a Fuzzy Integrated System (FIS) toolbox n a Matlab/Smulnk model (Fg. 3). Fg.3: FIS toolbox and Smulnk Model for BIPVT Prelmnary smulaton results suggest that there s scope to use F control n Matlab/Smulnk for developng predctve control strateges based on weather condtons to mprove BIPVT system performance usng modelng n TRNSYS. REFERENCES [] Munar Probst, M. and C. Roecker. Towards an mproved archtectural qualty of buldng ntegrated solar thermal systems (BIST). Solar Energy, (9): p [2] Schalkwjk, M.V., Opportuntes for PV n buldngs Results from the PV Catapult project [3] J, J, et. al. Effect of flow channel dmensons on the performance of a box-frame photovoltac/thermal collector,. Proceedngs of the Insttuton of Mechancal Engneers, Part A, Journal of Power and Energy,, 2006 Vol. 220, (No. A7): p. pp [4] Chow, T.T., e, W., J, J. An expermental study of façade-ntegrated photovoltac/water-heatng system. Appled Thermal Engneerng, (): p [5] Kang, M. C., Kang, Y.., m, S.., Chun, W. Numercal analyss on the thermal performance of a roof-ntegrated flat-plate solar collector assembly. Internatonal Communcatons n eat and Mass Transfer, 2006 Vol. 33: pp [6] Medved, S., Arkar, C., Cerne, B. A large-panel unglazed roof-ntegrated lqud solar collector--energy and economc evaluaton. Solar Energy 2003, 75(6): [7] J Bartelsen, B., et al., Elastomer-metal-absorber: development and applcaton. Solar Energy, (4-6): p [8] Colon, C., Merrgan, T., Roof ntegrated solar absorber: the measured performance of nvsble solar collectors. In: Proceedngs of ASES Natonal Solar Energy Conference, Washngton, DC, 200. [9] Anderson, T. Investgaton of Thermal Aspects of Buldng Integrated Photovoltac/Thermal Solar Collectors. PhD Thess, [0] Duffe, J.A., Beckman, W. A Solar engneerng of Thermal Processes. John Wley and Sons Inc., New York, 2006, 3rd edton. [] JA. Candanedo, B. O Nell, S. Pantc, A. Athents, Studes of control strateges for he Concorda solar house, n: Proceedngs of the 2nd Canadan Solar Buldngs onference, Calgary, Canada, 2007.