Predicting envelope and micro cogeneration design conditions for future climates.
Growing concerns about global climate change and green house gas (GHG) emissions have impacted many building sectors including the residential sector, which represents a large portion of the national energy consumption in many countries. Combating some of these challenges has often focused on improving energy efficiency or otherwise altering operation to reduce emissions through a wide range of strategies. Some of these strategies have focused on improvements to the building (e.g. retrofits) while others have focused on new technology developments including micro cogeneration or micro combined heat and power (mCHP) (Peacock and Newborough 2005). While much of the work in building research and new technologies have focused on how GHG emissions can be reduced through greater efficiency and improved performance, there is also a growing area of research dedicated to understanding how climate change is expected to impact buildings and new technologies. This is notably a challenging task as many studies rely on future climate predictions from different emission scenarios caused by a range of factors including technological advances, population growth and economic activity, creating high uncertainty (de Wilde et al. 2008; Gaterell and McEvoy 2005). However, the Intergovernmental Panel on Climate Change (IPCC) predicts an increase of 1.4-5.8[degrees]C (2.5-10.4[degrees]F) between 1990 and 2100 in surface temperatures and predictions of future climate scenarios have become increasing sophisticated since then (Frank 2005).
The first growing area of interest is understanding how climate change will impact existing buildings, particularly durability. In a recent work, Sehizadeh and Ge assessed two different retrofits that are being employed to enhance building envelope efficiency of two different typical Canadian wall assemblies (Sehizadeh and Ge 2016). Retrofitting of existing buildings for improved energy efficiency is becoming increasingly common in building codes worldwide (Gaterell and McEvoy 2005). One example includes the German PassiveHaus standard aimed at reducing energy consumption by 90% in dwellings (Sehizadeh and Ge 2016). One study on retrofitting found that improvements in insulation would reduce the annual energy consumption for heating by 53% under future climate scenarios (Gaterell and McEvoy 2005). Strategies for improvement under future climate conditions were also discussed by Holmes and Hacker (Holmes and Hacker 2007). Unfortunately, Sehizadeh and Ge indicate that the upgraded wall assemblies to the PassiveHaus recommended level actually reduces building durability. The durability performance was assessed in terms of the biodegradation risk of the plywood sheathing and the frost damage on the brick outer layer.
Other durability concerns have also been studied. Nijland et al. found that the number of freeze thaw cycles of porous materials would decrease in future climates due to elevated temperatures (Nijland et al. 2009). However, biodegradation of building materials was an increasing concern under these same conditions. Similar to Nijland, Grossi et al. found that frost damage to porous stones would likely decrease due to elevated temperatures (Grossi et al. 2007). Kolio et al. concluded that greater wind driven rain would likely increase on facades due to a combinations of temperature, precipitation and windiness (Kolio et al. 2014). Nik et al. found that mould growth risk would increase in future climates (Nik et al. 2012). Cultural heritage built with Carrara marble was found to be under increasing risk due to thermal stresses within the marble under future climate conditions, especially in the Mediterranean Basin (A. Bonazza et al. 2009). Increasing CO2 concentrations are also expected to increase surface recession of buildings and monuments with carbonate stone via the kurst effect (Alessandra Bonazza et al. 2009).
A second area of interest is how climate change will impact building technologies. Fewer examples exist of how residential building technologies will be impacted by climate change although there is some discussion for office buildings (Jenkins et al. 2008). However, it is well documented that the energy used for heating is expected to decrease while the energy for cooling is expected to increase much more (Frank 2005; Gaterell and McEvoy 2005; Wang et al. 2010; Zmeureanu and Renaud 2008). Christenson et al. predict a decrease in heating degree-days of 13-87% between 1975 and 2085 and an increase of up to 2100% in cooling degree-days for the same period (Christenson et al. 2006). Although not related to climate change, electrical energy usage for appliance and consumer electronics in residential buildings has also shown continued growth (U.S. Energy Information Administration 2011). Growing evidence of these expected changes will impact technology that is developed to meet heating, cooling and electrical needs within residential buildings. This is of particular interest for mCHP technologies which are currently being developed for residential buildings because unlike traditional separate generation of heat via a boiler or furnace and electricity via the electrical grid, these systems cogenerate both heat and electricity simultaneously. At times, mCHP technology is also used with an absorption chiller for micro combined cooling, heating and power (mCCHP) or trigeneration. In terms of design, future climates and living patterns are expected to impact each of these loads which can alter the performance of mCHP systems as the loads change. For example, the often noted thermal-to-electric ratio of heat-to-power ratio will vary because of changes in the heating, cooling and base electrical loads in residential buildings.
As noted by Sehizadeh and Ge, limited studies of envelope durability under future climates exist for North American climates. To this end, a portion of the study by Sehizadeh and Ge was first repeated using an envelope model employed in the Coupled Heat, Air, Moisture, and Pollutant Simulation in Building Envelope Systems (CHAMPS-BES) software in current and future climate predictions for 2020, 2050 and 2080. With the model predictions validated relative to the case study, the changes in envelope heat gain and loss were used to assess changes in the heating load for the retrofitted building. The loads were used to assess changes in the buildings heat-to-power ratio and its impact on mCHP technology is discussed.
One building envelope studied by Sehizadeh and Ge was assessed (Sehizadeh and Ge 2016). The envelope chosen represents one of the more common wall assemblies used in Montreal area and is considered to be among the best opportunities for retrofitting in that city. The envelope is a common Post-War wood frame construction. The base envelope is retrofitted with a double stud assembly having an 89 mm (3.5 in) gap between the studs filled with fiberglass batt insulation. Both the base and retrofitted construction were modeled in CHAMPS-BES software and assessed under current and future weather conditions for Syracuse, NY. The future climate data was generated using the Climate Change Weather File Generator program 'CCWeatherGen' recently developed by the Sustainable Energy Research Group at the University of Southampton (Jentsch et al. 2013). The generally accepted General Circulation Model HadCM3 and IPCCs A2 emissions scenario are used in this model (Nakicenovic and Swart 2000). Syracuse, NY weather data was taken at Syracuse Hancock International Airport using the TMY3 data set as the base year. Climate conditions specified for the envelope model included temperature, relative humidity, wind direction, wind velocity, direct and diffuse solar radiation, horizontal rain flux, cloud cover and atmospheric pressure.
The durability of the base and retrofitted wood assembly were assessed based on the biodegradation risk of the plywood sheathing. While there are many ways to make this assessment, co-occurrence of relative humidity above 80% and temperature above 5[degrees]C (41[degrees]F) (RHT) was used as described in other work (Sehizadeh and Ge 2016). The relative change in the heat flux through the building envelope are also assessed for the base and retrofitted assemblies.
After assessing the impact of climate change on the durability and energy performance of one retrofitting strategy for the building envelope, the design heating load was assessed based on the heat balance approach (McQuiston et al. 2005). Details of the whole building construction are found in Table 1. The 99% temperature for heating load calculations is not defined for future climates. Instead, the coldest annual temperature for the TMY3 data set, 2020, 2050 and 2080 was taken as the design point temperature for heating load calculations. The heating load (Q) was calculated based on equation (1). Here Ui is the heat transfer coefficient and Ai is the surface area for each of the i different wall constructions including walls, roof, fenestration and basement. The temperature difference across the assembly ([DELTA]T) is the difference between the indoor setpoint temperature and the outdoor design point temperature. The indoor set point is based on thermal comfort considerations and is set to 22[degrees]C (71[degrees]F) (ASHRAE 2013). Using this method the design point heating load is calculated for the building. Consideration of infiltration was not included in this initial assessment. Future work will include this impact in order to improve the accuracy of the results (Janssen, Pearman, and Hill 1980).
Q = [[summation].sub.i][U.sub.i][A.sub.i] x [DELTA]T (1)
With the heating load calculated, the implications for mCHP technology are assessed based on how the heat-to-power ratio changes.
RESULTS AND DISCUSSION
The simulated weather files indicate a 3-6[degrees]C (5-11[degrees]F) temperature rise throughout the year for the base year compared to 2080 while the relative humidity is expected to decrease by 10-12% during the summer months. Wind speed and solar radiation varied little according to the model while the annual precipitation is expected to increase by just less than 14%. The CHAMPS-BES model was used to simulate the TMY3 year conditions during the summer for both the basic wood frame construction and the retrofitted construction. Figure 1 shows that the temperature and relative humidity profiles for both cases. The results indicate that the relative humidity is actually higher throughout most of the insulated section with the wood studs reaching only a slightly higher relative humidity than the base case year. The plywood biodegradation risk was assessed using the RHT criteria by spatially averaging the temperature and relative humidity in the plywood sheathing. The results are shown in Figure 2 which indicate that the average relative humidity of the plywood actually increases for the retrofitted assembly relative to the base assembly during the spring and fall months, but is relatively consistent with the base case during the summer and winter months. This presents an increased risk because the temperature is above 5[degrees]C (41[degrees]F) during the spring and fall months and the relative humidity is above 80% for a greater period of time.
Figure 3 shows the change in heat flux for the base assembly and retrofitted assembly for the TMY3 data, 2020, 2050 and 2080. As expected, the heat flux from the assembly decreases during the winter months with the heat flux into the space actually increases during the summer. These results correlate with many other studies which predicted these trends. The retrofitted assembly performed better with the overall magnitude of the heat fluxes being much closer for each year simulated. However, the challenge is the disparity that exists between the results in Figure 2 and 3 which indicate enhanced thermal performance of the building envelope, but decreased durability based on biodegradation risk of the plywood sheating layer.
The results in Figure 3 demonstrate that the envelope heat flux during the heating season will decrease in future climates for both the base assembly and retrofitted assembly, but the base assembly will see a larger decrease in heat flux than the retrofitted assembly. During the cooling season, both the base and retrofitted assemblies have an increase in heat flux into the building in future climates resulting in an increase in the cooling load. Again, the total change for the retrofitted assembly is smaller than for the base assembly. These changes are plotted in Figure 4 below. The wide range between the base and retrofitted results indicates the potential impact the retrofit can have on a buildings thermal performance in future climates. With an understanding of the envelope performance in future climate conditions, the overall building was assessed for changes in the heating load. For the residential building described in Table 1 the heating load decreases from 6,026 W (20,560 Btu/h) to 4,900 W (16,720 Btu/h) between the TMY3 base year and 2080. Taking the TMY3 data as representative of the year 2000, there is a decrease of 0.2% per year in the heating load. The change is relatively small because the residential building is highly insulated, but it would be larger for older construction that is not well insulated. Cooling load calculations are notoriously more challenging as the steady state nature of Eq. 1 cannot be applied like it can for the heating load. While it is clear from Figure 3 and 4, the percentage change in the cooling load requires a more detailed building simulation performed under future climate predictions. More work is needed in this area to better predict the overall change in the heating and cooling load for the space.
While the change in heating load is small (0.2% per year), the potential challenge for mCHP technology comes when the other loads are also considered. Current trends indicate an increase in residential electric usage for appliances as well as for electric based cooling technologies operating in future climates. The combination of these two electric loads may cause the heat-to-power ratio for mCHP technology to converge as shown in Figure 5. However, the introduction of micro trigeneration technologies could also have the opposite effect as the cooling load will be provided by absorption refrigeration and is therefore thermally based. The heat generated in the prime mover will then be used for refrigeration during the summer months and the impact on the heat-to-power ratio may not be very significant. The implication is that different mCHP technologies may inherently be favored in the future do to the change in the heat-to-power ratio of the building. For example, fuel cell mCHP is known for a low heat-to-power ratio due to high electrical efficiency while small Stirling engines have a higher heat-to-power ratio due to low electrical efficiency (Penht et al. 2006). A more detailed study is needed to better predict the potential convergence of the heat-to-power ratio and the impact on current developments in the mCHP area. Furthermore, this discussion has primarily been limited to a prediction of the heat-to-power ratio for design conditions. The heating and cooling load are clearly very dynamic on an annual basis, as demonstrated in Figure 3, and on a daily basis with the peak heating load typically occurring in the early morning hours in the heating season and the peak cooling load often occurring in the afternoon. These dynamics are not represented by Figure 5, but do need to be included for a thorough understanding of the heat-to-power ratio changes in future climates.
Concerns over global climate change have called for many new technologies and building retrofits that are able to reduce GHG emissions and improve energy efficiency. While the potential for reducing climate change has been discussed as a motivation for these improvements, the impact of climate change on buildings and HVAC technology is a more recent area of investigation. A range of studies looking at changes in the heating and cooling loads, wind driven rain, weathering and durability have resulted in a variety of predictions. The work performed in this study confirms that retrofits in the Northeastern United States climate can improve energy efficiency of the building envelope through improved insulation, but may actually decrease the envelope durability due to increased biodegradation risk of the plywood sheathing. Retrofits should be assessed and performed with long term durability in mind otherwise more frequent maintenance and replacement may occur. The impact of mCHP technology on climate change is also of interest because mCHP has been identified as one technology that may help reduce GHG emissions from the residential sector. However, the impact of climate change on mCHP systems is an area that needs more investigation because simulatenous changes in heating, cooling and electrical loads may dictate which prime movers are best suited to meet future loads. More detailed analysis is needed to better predict the possible changes to the heatto-power ratio of residential buildings in future climates.
This material is based upon work supported by an agreement with Syracuse University awarded by its Syracuse Center of Excellence in Energy and Environmental Systems with funding under prime award number DEEE0006031 from the US Department of Energy and matching funding under award number 53367 from the New York State Energy Research and Development Authority (NYSERDA) and under NYSERDA contract 61736. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. 1247399 as well as the ASHRAE Graduate Student Grant-in-Aid.
NOMENCLATURE Q = Thermal capacity [A.sub.i] = Surface area of ith enclosure [U.sub.i] = Overall heat transfer coefficient of ith enclosure [DELTA]T = Temperature difference between indoor setpoint and design heating load temperature
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Ryan J. Milcarek
Student Member ASHRAE
Jeongmin Ahn, PhD
Student Member ASHRAE
Jianshun Zhang, PhD
Student Member ASHRAE
Ryan Milcarek, Shaun Turner and Rui Zhang are graduate students in the Department of Mechanical & Aerospace Engineering, Syracuse University, Syracuse, NY. Jeongmin Ahn is a Associate Professor in the Department of Mechanical & Aerospace Engineering, Syracuse University, Syracuse, NY. Jianshun Zhang is a Professor in the Department of Mechanical & Aerospace Engineering, Syracuse University, Syracuse, NY.
Caption: Figure 1: Horizontal cross-section of the wood frame assembly for the base case A) temperature ([degrees]C) and B) relative humidity distribution and the retrofitted assembly C) temperature and D) relative humidity distribution.
Caption: Figure 2: Spatially averaged annual temperature and relative humidity in the plywood sheathing for the base and retrofitted assembly.
Caption: Figure 3: Annual heat flux (W [m.sup.-2]) for the A) base assembly and B) retrofitted assembly.
Caption: Figure 4: Change in peak heating and cooling season heat flux (W [m.sup.-2]) for the base assembly and retrofitted assembly.
Caption: Figure 5: Expected trends for the residential building heating, cooling and electrical loads and its impact on the mCHP heat-to-power ratio.
Table 1. Heat Transfer Coefficients and Surface Areas of Envelope Components. Envelope component Ui Glazing 1.99 W/[m.sup.2] K (0.35 Btu/hr [ft.sup.2] F) Walls 0.40 W/[m.sup.2] K (0.07 Btu/hr [ft.sup.2] F) Roof 0.45 W/[m.sup.2] K (0.08 Btu/hr [ft.sup.2] F) Basement floor 0.68 W/[m.sup.2] K (0.12 Btu/hr [ft.sup.2] F) Floor 0.62 W/[m.sup.2] K (0.11 Btu/hr [ft.sup.2] F) Below grade 0.23 W/[m.sup.2] K (0.04 Btu/hr [ft.sup.2] F) Door 1.14 W/[m.sup.2] K (0.20 Btu/hr [ft.sup.2] F) Envelope component Ai Glazing 13.0 [m.sup.2] (139.92 [ft.sup.2]) Walls 155.1 [m.sup.2] (1669.32 [ft.sup.2]) Roof 19.3 [m.sup.2] (208 [ft.sup.2]) Basement floor 39.0 [m.sup.2] (420 [ft.sup.2]) Floor 30.7 [m.sup.2] (330 [ft.sup.2]) Below grade 65.4 [m.sup.2] (704 [ft.sup.2]) Door 11.6 [m.sup.2] (124.73 [ft.sup.2])
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|Author:||Milcarek, Ryan J.; Ahn, Jeongmin; Turner, Shaun; Zhang, Jianshun; Zhang, Rui|
|Date:||Jan 1, 2017|
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