1、 oabRTppbbttUccCO u KKK12Role of Radiant Panel Heating and Cooling in Net Zero Energy Buildings Birol Kilkis, Ph.D. Fellow ASHRAE ABSTRACT For a successful and sustainable net-zero energy building, a careful match and balance between the supply exergy and demand exergy is essential. Especially in lo
2、w-exergy buildings, radiant panels offer unique advantages primarily due to their moderate temperature and particularly low-exergy demands. A parametric study is included in the presentation in order to better quantify the benefits of radiant panel systems in view of energy and exergy benefits. A ne
3、w solar tri-generation system is presented, which acts like a solar PV on the outside and acts as panel cooling in the inside. This system replaces non-load bearing external wall elements and offer substantial energy and carbon emission efficiencies. INTRODUCTION The overall success of radiant panel
4、 heating and cooling depends on exergy of the type of energy source used in the building. For example, if fossil fuel is the energy source, the exergy efficiency of a radiant panel heating system in a “low-exergy building” may prove to have lower rational exergy management efficiency than other hydr
5、onic or even forced-air systems. In order to correct this apparent issue, a radiant panel system, which is a low-exergy demanding system must be coupled with low-exergy energy resources like low-intensity renewables or waste heat or cold. When the objective is to reduce carbon emissions especially f
6、or net-zero energy buildings, usually solar and wind energy resources are prioritized. For example, solar collectors are used for space heating in several countries in conjunction with radiant panel systems. In fact both make a good coupling due to the fact that moderate temperatures that may be del
7、ivered with solar collectors make a good match with moderate water temperature requirement of radiant panel systems. This match also reduces carbon emissions, because such a good temperature match also replicates itself in exergy match. According to the Rational Exergy Management Model (REMM), the c
8、ompound carbon emission per hour is given in Equation 1. Here, cbis the carbon equivalency of the energy source and is the first-law efficiency of the system. The subscript “ p” stands for the power plant that the building is attached via the grid, with an overall power delivery efficiency of T. The
9、 term Ubstands for the overall heat transfer coefficient of the building envelope (W/K). (1) The term Ris the REMM efficiency (Kilkis, S. 2009). The second term represents avoidable carbon emissions due to exergy imbalances. Equation 3 is the demand exergy for unit load and Equation 4 is the supply
10、exergy based on the supply temperature Tfof the energy source in kelvin. Trefis the environment reference temperature. supHHdemR (2) Birol Kilkis is a professor at Bakent University and Head of Energy Engineering Graduate Program, Ankara LV-11-C073602 ASHRAE Transactions2011. American Society of Hea
11、ting, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.(11
12、.7%) 0.11735.017.085.055.01111 CHPErefCHPECHPHrefCHPHPESPVTKK arefdemTT1H(3) frefTT1supH(4) DEVELOPMENT OF A NEW RATING PARAMETER Radiant panel heating technology is recently coupled in many countries with PVT (Solar photovoltaic and heat). Radiant panel cooling is also possible now by PVTC (Solar P
13、hotovoltaic, thermal, and cooling) systems. Speaking about PVT, a new rating parameter, which is not available yet, was developed in this study in order to accurately quantify benefits of coupling PVT and PVTC technology with radiant panel heating and cooling concept. This new rating parameter combi
14、nes environmental benefits and fuel savings benefits. To begin with the development of the new parameter, PES equation of the EU Directive 1 on co-generation, aka Combined Heat and Power (CHP), may be adopted to form a trial form of PESPVTsolution: (5) The reference values CHPEref and CHPHref are gi
15、ven in the same Directive. Because the solar PVT is on-site, the reference efficiency values must be selected accordingly (on-site availability of energy and power). The overall electric power generation, distribution, and transmission efficiency is about 0.35. In a typical PVT system, solar- therma
16、l efficiency is about 0.55 and solar-power efficiency is typically 0.17. The condensing-boiler reference efficiency is about 85% on average load. Yet, the result obtained by the above trial form of PESPVTequation and the moderate fuel savings percentage does not represent actual benefits of a PVT sy
17、stem. This is primarily because it does not factor in the rational exergy management efficiency, namely R, which is a strong metric about environmental benefits of alternative energy systems. A recent study has modified Equation 5 in terms of R. The result is PESRPVT2: (6) Here, RPVTfor PVT is appro
18、ximately equal to 0.34. The reference value, namely Rref is 0.2024 3. The result is not successful again and in fact, meaningless, because what it tells us is that one may save only 18% of fossil fuel “used.” In a solar PV or PVT system, no fossil fuel use is involved unless a fossil fuel-hybrid sys
19、tem is employed. Above two tries to factor in benefits of radiant panel systems to PVT and PVTC systems are not successful. This condition justifies the need for a new rating parameter for alternative energy-combined heat and power systems. The new parameter is a simple modification of Equation 6. T
20、hen actual fuel savings of a solar PVT system, which supplies all power and heat demand (QA/Q =1) is: (7) Here, QAis the sum of all heat (and or cold) supplied by a solar and or any other alternative energy resource. Q is the total heat and power demand of the building. If for example, in a hybrid (
21、Boiler and grid power integrated solar PVT) system QA/Q is 0.45, then Equation 7 gives a fuel savings ratio of 0.4775 (47.75%) for the same example. In a further refinement of Equation 7 exergy difference between heat and power is recognized: (18%) 0.1834.022024.0235.017.085.055.0112211 u u RPVTRref
22、RPVTCHPErefCHPECHPHrefCHPHPESKK (100%) 11 RPVTARPVTRAPESQQPESPES 2011 ASHRAE 603(8) In Equation 8, Q terms are broken down into heat and power components and into their respective exergy values, namely 0AH, 0AE, 0HH, 0EE. Exergy of electric power, namely 0AEand 0EEwere taken unity. Compare a PVT sys
23、tem with a condensing-type “wall-mounted combi” heater used on-site (in the building) in summer for domestic hot water supply, where the electric power is supplied from the national grid. PESRcombivalue in this case is negative even the condensing boiler may have a peak on-site thermal efficiency of
24、 0.95. This is primarily because; it does not generate power at all. Actually a “high-efficiency” boiler wastes fuel rather than saving it 4, 5. Therefore, the overall energy savings of a PVT system compared to natural-gas condensing boiler is 108%. PVTCI CONCEPT In spite of their environmental bene
25、fits, the problem with PVT is the fact that there is not much thermal load during summer season. Instead cooling loads are dominant. Therefore, in order to avoid wasting solar heat from PVT panels, it needs to be utilized for generating cold downstream the process, like by an absorption unit or by a
26、 metal-hydride system. Although these are technically feasible, the system mechanics becomes quite complicated and operation and installation costs may be uneconomical. In this study an electronic version of the concept has been developed. The system comprises an integrated, sandwiched unit that emp
27、loys the electro-thermal effect both for heating and cooling: IBPVTCI (Building Integrated Photo-voltaic, thermal, and cooling) system. The heat generation is multiplied by the absorbed heat from the building while electronically cooling the building using thermo-electric cooling (TEC) modules. Beca
28、use comfort cooling is performed mainly to satisfy solar gains of a building in summer, the new PVTCI heat output is even more than the solar radiation on the PVTCI. Figure 3 Basic Concept of the PVTCI System 6. The same sheet thermally insulates both modules, because this commercially-available she
29、et is practically a perfect insulator along its thickness direction. This conductive sheet performs in such a manner that while the heat is transferred to a proper heat sink at the demand point, it cools the PV module and maintains the proper temperature difference across the TEC module. During a ty
30、pical operation, the solar PV module generates electric power at its optimum performance level, because it is cooled. With a simple control, this solar power may be optimally split between the power need of the building and the power need of the TEC module. When the TEC module is exposed directly or
31、 indirectly (by a second thermally conducting sheet layer to the indoor space), it electronically heats or cools the indoor space primarily by thermal radiation and secondarily by natural convection through its exposed surface, depending upon the polarity of the dc power supplied by the PV module, w
32、hich is controllable. In the cooling mode, TEC module absorbs heat from the indoor space and transmits it to the same heat conducting sheet between the TEC module and the PV module. Thus this system multiplies the heat gain that may be usefully utilized in the same indoor space or other building zon
33、es. If the cooling load is the Electric Power Heat Conducting FilmSOLAR PV MODULE TEC MODULESpace Cooling or HeatingHeat DHW Heat Activated Cooling RPVTEHHHAEAHHARPVTRAPESQQQQPESPES 1HH (-8%) 0.0806.022024.0235.0095.095.0112211 u u RPVTRrefRcombiCHPErefCHPECHPHrefCHPHPESKK604 ASHRAE Transactionsdomi
34、nant load, part of this heat may be further utilized in a heat activated cooling system like a metal-hydride system. During the space heating season in winter, a simple switch of the polarity makes the same TEC module a radiant space heating module. In this case, the heat conducting film may bring t
35、he PV heat into the indoor space. DESIGN OF PVTCI The concept was engineered in an optimized manner and a prototype was manufactured with an emphasis of the possibility of using this concept on building facades, which may receive solar energy as a building element with an optimized inclination. This
36、 modular system may be cast into non-load bearing bricks while it also acts like an insulating material. This introduces a fourth useful function to the concept, namely thermal insulation of the building envelope (PVTCI). Figure 4 shows the wall application schematics. This additional thermal insula
37、tion reduces thermal loads of the building and results in a better energy and exergy balance and match between PVTCI outputs and building demands. According to this figure, the heat exchanging medium is the heat conducting sheet that may be connected to a hydronic circuit at the end of the walls; on
38、e supply and one return circuit that transfers the absorbed heat to demand points or to other heat activated cooling systems. The inclination angle, . from the vertical and rotation angle, from the normal to the facade is optimized for minimum shading of adjacent modules and maximum solar gain. Inte
39、rnal gaps thus formed are filled with conventional insulating material. This application also satisfies sound and vibration insulation requirements. Figure 4 Building integrated version of PVTCI 7. APPLICATION A prototype was developed and manufactured 8 for testing the performance of a unit panel.
40、The basic design, of a single PVTCI module is shown in Figure 5. Here, the conducting sheet is thermally riveted to a metal plate at one end. The metal plate is connected to a hydronic copper pipe, in which water circulates to transfer the heat from the module to points of use and thermally sinks bo
41、th PV and TEC modules. The calculated performance of the PVTCI system under ideal conditions with optimally sized and selected PV and TEC modules is summarized in Figure 6. In this calculation, the cooling COP of TEC is 0.70, PV efficiency is 0.2 9. Typically, the solar power s equally split between
42、 the building and TEC modules. For an incident total solar insolation on a 1 m2of PVTCI panel surface area of 160 W, the breakdown of the useful outputs is given below. o Total solar electric power supply = 32W. o 16W to electric demand (Net power supply to the building) o 16W to TEC (For internal u
43、se in PVTCI modules) x TEC cooling capacity = 11.2W at an indoor DB air temperature of Ta= 295K (22oC) x Solar heat capacity = 115.2W at Theat= 343K (70oC). x Total useful output: 115.2W+16W+11.2W+Heat gain from comfort cooling The first-law efficiency of the module under ideal conditions is; 142.4W
44、/160W = 89%, excluding the heat gain from indoors during comfort cooling. 2011 ASHRAE 605 44832.115.27315.27310578.1 u ppaTECTECtAUSTttAQFigure 5 A Single Solar Tri-generation (PVTCI) module 8. COOLING SURFACE ENHANCEMENT A thermal pinch exists between the solar PVTCI system and its radiant surface
45、exposed to the indoor space to be electronically heated or cooled. This pinch occurs on the TEC surface: TEC surface area, namely ATEC is not sufficient to transfer heat between indoors and the TEC surface and eventually in the PVTCI module. The total heat transfer (Natural convection and radiation)
46、 between a vertical radiant panel (TEC) surface and the indoors, namely QTECat given thermal conditions is calculated by Equation 9 10. In this equation, AUST is the area-average temperature of unconditioned indoor surfaces. This equation is valid for both sensible heating (Negative QTEC) and coolin
47、g (Positive QTEC). PVTCI module may also heat indoors during winter by a simple polarity switch of the dc power supply and disengaging the heat sink. The coefficient 1.78 and the power 1.32 may slightly change according to the angle of inclination and spacing of the PVTCI modules. The heat transfer
48、coefficient regarding natural convection may develop into the forced-convection regime at the presence of another installed HVAC system. For typical surface dimensions of a single TEC module (0.05 m x 0.05 m), QTECis about 0.18W if tpis 18oC, AUST is 28oC and tais 24oC. But the same TEC module is capable of absorbing 5W of heat in the cooling mode under
