1、Exergy Metrication of Radiant Heating and Cooling Birol Kilkis, Ph.D. Fellow ASHRAE ABSTRACT In low-energy and low-exergy buildings, radiant heating and cooling are known to have a good track record of energy efficiency in terms of the first-law of thermodynamics. With recent concerns of global warm
2、ing and environmental degradation however, quality of energy aka exergy balance between the supply and demand in terms of the second-law of thermodynamics is also becoming a dominant factor. Especially in radiant panel heating and cooling a new metrication that may guide the designer to choose the r
3、ight supply chain for both quality and quantity of energy is needed in order to realize the benefits of radiant heating and cooling and to recognize potential limitations. INTRODUCTION Radiant panel heating and cooling systems are temperature-controlled indoor surfaces that maintain sensible thermal
4、 comfort primarily by controlling the operative temperature (OT) and the Mean Radiant Temperature (MRT). ASHRAE Hand Book-HVAC Systems and Equipment, Chapter 6 defines a radiant panel heating and cooling system as follows (ASHRAE 2008): “A temperature-controlled surface is called a radiant panel if
5、50 % or more of the design heat transfer on the temperature-controlled surface takes place by thermal radiation.” On the equipment performance side, such a condition for sensible comfort heating may be satisfied even at quite low dry-bulb (DB) indoor air temperatures (ta). Radiation and convection h
6、eat transfer flux from a radiant floor heating panel maintained at an effective surface temperature of tpfor example are (ASHRAE 2008): 448)15.273105 AUSTttqppru (1) Here, AUST is the area-averaged temperature of uncontrolled indoor surfaces, and it depends on design indoor temperature ta, outdoor t
7、emperature to, number of exposed sides, building insulation, degree of fenestration etc. 31.113.2apcttq (2) crrqqqPR PR 0.5 (3) Here, PR is the radiant heat transfer to total heat transfer ratio at the panel surface. If for example, AUST in panel heating about 1oC lower than the DB air temperature,
8、and tpis 29oC (maximum permissible temperature in floor heating), the variation of PR with tais simply given in Figure 1. Birol Kilkis is a professor at Bakent University and Head of Energy Engineering Graduate Program, Ankara LV-11-C00864 ASHRAE Transactions2011. American Society of Heating, Refrig
9、erating 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.56.346.402.8 MR
10、TOTta Figure 1 Variation of PR with ta. Figure 1 shows that the threshold value of 50% for PR may be satisfied at low tavalues. This has an important implication on net-zero energy and low-exergy buildings, because sensible comfort loads (Q) in kW primarily depend on ta. For a given building of know
11、n thermal properties, Q may be simplified in the following form: )(oabttUQ (4) Here, Ub is the overall heat loss (gain) coefficient of the building under consideration (kW/K). Equation 4 indicates that Q may be reduced by decreasing ta(increasing in cooling). Of course this process is subject to the
12、rmal comfort requirements and constraints. On the human thermal sensation side, ASHRAE comfort diagram associates human thermal comfort with Operative Temperature (OT) instead of ta. In other words, the primary human comfort temperature is OT, and tais a dependent variable. For typical indoor condit
13、ions (an active person in still air with M = 1.1 met and the air velocity less than 0.2 m/s, this relationship becomes (Kilkis, B 2010) : (5)Because MRT that is controlled by radiant panels is higher in space heating for example, when compared to other heating systems, tavalue may be lower for a giv
14、en OT and the same thermal comfort (ASHRAE 2005). This dependence is given in Figure 2, after re-writing Equation 5 for the same case: MRTOTta25.125.2 (6) In space cooling, the same relationship shows that tamay be higher than other cooling systems for the same sensible comfort. Therefore, it may be
15、 concluded that as common sense also tells, DB indoor air temperature may be lower in heating and higher in cooling for the same thermal comfort, while sensible comfort loads of a building may approach to net-zero and low-exergy conditions for green and high-performance buildings. Fortunately a radi
16、ant system just does that. For example, MRT value may be directly controlled by radiant panel surface temperature controls (ASHRAE 2003). In other words, while radiant panels address primarily OT and thus human comfort, taaddresses building thermal loads that may be independently minimized provided
17、that human comfort is not compromised. y = 0.0149x + 0.29280.40.450.50.550.60.650.70.7515 17 19 21 23 25 27PRDB Air Temperature, ta(oC)RADIANT 2011 ASHRAE 65Figure 2 Variation of tafor a given OT value with MRT, which is controlled by radiant panels. In addition radiant panels do indeed improve the
18、human comfort sensation, because human body is best satisfied at a high radiant to convective heat transfer split, provided that radiant asymmetry, hot panel surface restrictions are satisfied. Furthermore human body exergy loss in general calls for high MRT values compared to ta(Kilkis, B 2010). TH
19、EORY In terms of the first-law of thermodynamics, while radiant panels permit the use of lower DB air temperatures for equal comfort and less energy demand according to Equation 4, carbon emissions decrease. On the other hand in terms of the second-law of thermodynamics the same is not necessarily t
20、rue:, against common sense, because radiant panels demand lower exergy due to ability to call for lower DB design air temperatures (See Equation 8) and operate at moderate fluid temperatures, the exergy supply and demand balance may deteriorate (Kilkis, B 2007). In fact this is a common issue for lo
21、w-exergy buildings unless low-exergy demand of the building is better balanced by low-exergy energy resources like waste energy and low-intensity renewables. Because the exergy balance-based carbon emissions depend upon exergy balance, an apparent dilemma exists for radiant panels. In order to expla
22、in and resolve this dilemma; energy and exergy-based carbon emissions must be simultaneously considered as well as the energy loads. Compound Carbon Emissions According to the Rational Exergy Management Model (REMM), the compound carbon emission per hour of the building operation under constant comf
23、ort load, according to the first-law and the second-law of thermodynamics is given in Equation 7 (Kilkis, S 2006-a): oabRTppbbttUccCO u KKK12(7) Here, cbis the carbon equivalency of the energy source used in the building and is the first-law efficiency of the system. The subscript “p” stands for the
24、 power plant that the building is attached via the grid, with an overall power delivery efficiency of T. The term Ris the new Rational Exergy Management Model (REMM) efficiency (Kilkis, S 2006-b). The second term represents avoidable carbon emissions due to unbalances between demand and supply exerg
25、y, 0demand 0sup, respectively for a given building and corresponding indoor/outdoor thermal conditions (Kilkis, S. and Kilkis, B. 2010): supHHdemR (8) oabarefdemttUTTu 1H(9) 151719212317 18 19 20 21 22 23taMRT, oCOT: 20oC66 ASHRAE TransactionsW0.8585kW/k20002831 KKE uu mrefrefaoabRTppbbTTTttUccCO293
26、12KKK oabttUE u supH (10) E is the unit exergy (kW/kW) of the energy supply to the building. For example, if the energy supply is natural gas at a flame temperature of 2000 K, then according to ideal Carnot cycle, E is: (1) Here, the environment reference temperature (Tref) was taken 283K. At this t
27、emperature all thermal processes may reach equilibrium in the nearby environment of the building. It may be the outdoor air temperature, ground temperature, or temperature of a lake or sea that may be present nearby the building considered. Then the Rational Exergy Management Method Efficiency may b
28、e re-written: arefRTTE11(12) Combining Equations 7 and 9 for a radiant panel heating system, one may conclude that energy and exergy savings is not a simple matter of reducing (or increasing in cooling) DB design air temperature, but a necessity of a careful optimization of ta. The same argument hol
29、ds true for radiant panel cooling. To exemplify the case, three scenarios were developed for radiant floor heating panels. The first scenario involves a grid-connected building that has a natural-gas fired boiler, radiant floor system. Ub values were parametrically analyzed as shown in Figure 3. In
30、this scenario, E is 0.8 kW/kW for the natural gas boiler. Building insulation level represented by Ub was parametrically changed from 0.4 kW/K to 0.8 kW/K. As seen in Figure 1 and Table 1, for the given other design and operating conditions, carbon emissions decrease with taalthough Rvalue decreases
31、 with ta. Figure 3 Scenario1: Base Case and Parametric Analysis of Carbon Emissions with Uband ta. The latter condition is true because the temperature difference between the natural gas flame temperature in the boiler and taincreases with a decrease in ta. The impact of taon carbon emissions (Slope
32、 of the lines on Figure 3) somehow decreases with decreasing Ub(Better insulation). All these results show that while a radiant heating panel system accommodates the design of lower tavalues, it contributes to the environment by enabling the decrease of compound carbon emissions, although its appare
33、nt Rvalue decreases.(13)0.005.0010.0015.0020.0025.0015 17 19 21 23kg CO2/hDB Air Temperature, ta(oC)Ub=0,8Ub=0,6 kW/K (Base Scenario)Ub=0,4E= 0,8 kW/kW cb= cp= 0,2 kg CO2/kw-h (Natural gas) b= 0,75; p=0,55; T= 0,7 OT = 20oC; to= -12oC 2011 ASHRAE 67The second scenario is a case where the building is
34、 heated by waste heat from a district energy system but it continues to be connected to the power grid. Figure 4 shows rather an unexpected result: when the building is improved towards a low-energy, low-exergy green building and much better insulation (Ubvalues are lower than in Scenario 1), radian
35、t panels do not contribute to decrease of carbon emissions, i.e. by lowering tawith radiant heating panels, compound carbon emissions seem to increase. This is due to the fact that COP value of energy supply systems becomes dominant in such improved building projects. In order to correct this issue,
36、 Equation 7 was simplistically modified as given in Equation 13. Here, the exponent m depends on the exergy supply system. For example a ground-source heat pump based district energy system, m is 0.4. Higher the m value higher will be the COP value and its impact on reducing carbon emissions. In Equ
37、ation 13, reference COP value is based on the condition of ta=20oC. In Scenario 3, Equation 13 with conditions given above was applied. Figure 5 now shows that radiant panel systems are advantageous also for low-energy, low exergy green buildings in reducing compound carbon emissions, if the dominan
38、t COP value of the exergy supply system is factored in to the new exergy metric. In fact this coincides well with green buildings where high COP (high m) systems like ground-source heat pumps are utilized. This research now quantifies earlier discussions about the energy and environment benefits of
39、coupling heat pumps with radiant panel systems (Kilkis, I. B. 1999). Table 1. Scenario 1 Calculations for Ub = 0.6. OT = 20oC. ToTrefE OT taTaUb%RCO2K K kW/kW KoC K kW/K - kg/h261,15 283,15 0,8 293 14 287,15 0,6 0,017 12,123 261,15 283,15 0,8 293 14,5 287,65 0,6 0,020 12,338 261,15 283,15 0,8 293 15
40、 288,15 0,6 0,022 12,553 261,15 283,15 0,8 293 15,5 288,65 0,6 0,024 12,767 261,15 283,15 0,8 293 16 289,15 0,6 0,026 12,981 261,15 283,15 0,8 293 16,5 289,65 0,6 0,028 13,194 261,15 283,15 0,8 293 17 290,15 0,6 0,030 13,406 261,15 283,15 0,8 293 17,5 290,65 0,6 0,032 13,618 261,15 283,15 0,8 293 18
41、 291,15 0,6 0,034 13,829 261,15 283,15 0,8 293 18,5 291,65 0,6 0,036 14,040 261,15 283,15 0,8 293 19 292,15 0,6 0,039 14,250 261,15 283,15 0,8 293 19,5 292,65 0,6 0,041 14,460 261,15 283,15 0,8 293 20 293,15 0,6 0,043 14,669 261,15 283,15 0,8 293 20,5 293,65 0,6 0,045 14,877 261,15 283,15 0,8 293 21
42、 294,15 0,6 0,047 15,085 261,15 283,15 0,8 293 21,5 294,65 0,6 0,049 15,292 261,15 283,15 0,8 293 22 295,15 0,6 0,051 15,499 261,15 283,15 0,8 293 22,5 295,65 0,6 0,053 15,705 CONCLUSIONS A new exergy and energy-combined metric was developed in order to better analyze and quantify the benefits of ra
43、diant panel systems. Because radiant panel systems can maintain the same or even better comfort sensation, as quantified in this paper, at lower (in heating) and higher (in cooling) indoor air temperatures and moderate fluid temperatures for distributing energy, energy and exergy savings is possible
44、 in all types of buildings using radiant panel systems. More the buildings in the building stock become low-exergy buildings with radiant panel 68 ASHRAE Transactionsheating and cooling, and/or chilled beams for example and low-energy type of buildings with better insulation for example, COP factor
45、for the energy and exergy supply source become dominant in the analysis. This makes it easier to reveal the energy and exergy benefits of radiant panel systems, including environmental benefits. Figure 4 Scenario 2: Well-insulated, Low-Exergy Low-Energy Building without COP metric. Figure 5 Scenario
46、 3: Well-insulated, Low-Exergy Low-Energy Building with high COP Equipment and Radiant Floor Heating System. For example, if the dependence of design ta on COP factor is not factored in to the new metric, advantages of radiant panel systems both for heating and cooling disappear. For this reason the
47、 metric was modified with a COP factor for radiant panels. In this case, the operating fluid temperature in a radiant panel system, which directly affects the COP, is directly proportional to ta. This relationship made easier the so-called modification of the metric. Once the metric was modified acc
48、ordingly, the benefits of radiant panels were better quantified. The results show that high-COP systems coupled with low-intensity renewable and waste energy systems make the best combination with radiant panel systems and render the least carbon emission rates for the high-performance, low-exergy building industry. This holds true both for heating and cooling b radiant panels. The new metric may also be used to predict the fuel consumption rates and provides background data for a complete economic analysis. It also enables the des
