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The '''building balance point temperature''' is the outdoor air temperature |
The '''building balance point temperature''' is the outdoor air [[temperature]] when the heat gains of the building are equal to the heat losses.<ref name=Utzinger>{{cite web|url=http://www.arch.ced.berkeley.edu/vitalsigns/res/downloads/rp/balance_point/balance_point_big.pdf |title=Vital Signs Curriculum Materials Project |last1=Utzinger |first1=Michael |last2=Wasley |first2=James |website=UC Berkeley |publisher=College of Environmental Design |accessdate=25 November 2014 |url-status=dead |archiveurl=https://web.archive.org/web/20120612033218/http://arch.ced.berkeley.edu/vitalsigns/res/downloads/rp/balance_point/balance_point_big.pdf |archivedate=12 June 2012 }}</ref> Internal heat sources due to [[electric light]]ing, mechanical equipment, [[thermoregulation|body heat]], and [[Sunlight|solar radiation]] may offset the need for additional heating although the outdoor temperature may be below the thermostat set-point temperature. |
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[[File: Balance_point_temperature_diagram.jpg |thumb|right|alt=Balance point temperature in reference to ideal indoor temperature and building heat gains and losses| Balance point temperature in reference to ideal indoor temperature and building heat gains and losses.]] |
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The building balance point temperature is the base temperature necessary to calculate [[heating degree day]] to anticipate the annual energy demand to heat a building. The balance point temperature is a consequence of building design and function rather than outdoor weather conditions.<ref name=Lechner>{{cite book |last=Lechner |first=Norbert |date=2009 |title=Heating, Cooling, Lighting: Sustainable Design Methods for Architects |location=Hoboken, NJ |publisher=John Wiley & Sons}}</ref> |
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[[File:Energy_gains_and_losses_through_building.jpg|thumb|right|alt=Internal and external heat gains and losses in a building.|Internal and external heat gains and losses in a building.]] |
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<br>Where: |
<br>Where: |
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*''t<sub>balance</sub>'' is the |
*''t<sub>balance</sub>'' is the balance point outdoor air temperature, given in °C (°F). |
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*''t<sub>Thermostat</sub>'' is the building thermostat |
*''t<sub>Thermostat</sub>'' is the building thermostat set-point temperature, given in °C (°F). |
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*''Q<sub>IHG</sub> is the internal heat generation rate per unit floor area due to occupancy, electric lighting and mechanical equipment. This internal heat generation is not constant due to variability in occupancy, lighting, and equipment operation schedule but is largely considered constant to a first order approximation. |
*''Q<sub>IHG</sub> ''is the internal heat generation rate per unit floor area due to [[occupancy]], electric lighting and mechanical equipment, given in W/m<sup>2</sup> (Btu/s/ft<sup>2</sup>). This internal heat generation is not constant due to variability in occupancy, lighting, and equipment operation schedule but is largely considered constant to a [[Orders of approximation#First-order|first order approximation]]. |
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*''Q<sub>SOL</sub>'' is the |
*''Q<sub>SOL</sub>'' is the building heat gain per unit floor area due to solar radiation, given in W/m<sup>2</sup> (Btu/s/ft<sup>2</sup>). This heat gain is not constant due to solar variability with time of day and year but is largely considered constant to a [[Orders of approximation#First-order|first order approximation]]. In winter, it is reasonable to assume ''Q<sub>SOL</sub>''=0''.'' |
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*''U<sub>bldg |
*''U<sub>bldg</sub>'' is the rate of [[heat transfer]] across the building envelope per degree temperature difference between outdoor and indoor temperature and per unit floor area, given in W/K/m<sup>2</sup> (Btu/s/°F/ft<sup>2</sup>). This heat transfer can vary due to variations of fresh air ventilation rate but is largely considered constant to a [[Orders of approximation#First-order|first order approximation]]. |
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This equation is simplified by assuming steady state heat transfer between the building and the environment and only provides an approximate building balance point temperature. The 2013 ASHRAE Handbook – Fundamentals, Chapter F18 provides more rigorous methodologies to calculate the heating loads in a nonresidential buildings. The ASHRAE heat balance method, for example, fully delineates the heat transfer through the inner and outer boundaries of the building wall by incorporating radiative (e.g. sun, indoor surfaces), convective (e.g. indoor and outdoor air), and conductive (e.g. inner to outer boundary) modes of heat transfer.<ref name=Utzinger /> |
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Internal load dominated buildings are usual compact with a low surface-area-to-volume ratio. These buildings usually have many rooms with only zero to one exterior walls in each room. Due to usually higher internal heat gains, internal load dominated buildings do not require passive solar heating (except in cold climates) but rather require passive or mechanical cooling. For large buildings, an internal load may not be able to sufficiently buffer envelope heat exchange throughout the building especially in perimeter regions. Large office spaces, schools and auditoriums are typical examples of internal load dominated buildings where the balance point temperature is set around 50oF (10oC) <ref name=Lechner />. |
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==Determination Methods== |
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[[File:Energy_signature_method_to_determine_balance_point_temperature.png|thumb|right|An example of determining a building's balance point temperature using the energy signature method.]]In real-world scenarios, the balance point may be determined in one of two ways. In the [[energy signature]] method, a plot is created mapping energy consumption against mean outdoor temperature. The point on the chart at which weather-independent and weather-dependent electricity or gas demand intersect is the balance point temperature. This method only works if large quantities of data on the building energy use are available, preferably on a daily resolution.<ref name=Lee>{{cite journal |last1=Lee|first1=Kyoungmi|last2=Baek|first2=Hee-Jeong |last3=Cho|first3=ChunHo |date=2014|title=The Estimation of Base Temperature for Heating and Cooling Degree-Days for South Korea |journal=Journal of Applied Meteorology and Climatology |volume=53|issue=2|pages=300–309 |doi=10.1175/jamc-d-13-0220.1|bibcode=2014JApMC..53..300L }}</ref> |
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In the '''performance line method''' multiple plots of energy consumption against [[heating degree day]]s (HDD) and cooling degree days (CDD) are created, using a range of '''balance point temperatures''' to calculate the degree days. Best-fit second-order polynomials of the form ''{{math|y{{=}}ax<sup>2</sup>+bx+c}}'' are then applied to the plots, which show various levels of curvature across the range of the data depending on the accuracy of the balance point temperature. In plots with overly high balance point temperatures the ''{{math|a}}'' variable is positive, resulting in an upward curve, while plots with low balance point temperatures curve downward due to a negative ''{{math|a}}'' variable. The plot in which ''{{math|a}}'' is closest to zero represents the most accurate balance point temperature. This method may be applied to buildings in which the availability of energy use data is less granular, perhaps only available on a weekly or monthly basis.<ref name=Day>{{cite journal |last1=Day|first1=A. R.|last2=Knight|first2=I. |last3=Dunn|first3=G.|last4=Gaddas |first4=R. |date=2003|title=Improved methods for evaluating base temperature for use in building energy performance lines |journal=Building Services Engineering Research and Technology |volume=24|issue=4|pages=221–228 |doi=10.1191/0143624403bt073oa|s2cid=111019051 }}</ref> |
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| ⚫ | Solar |
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A building's thermal characteristics may be described as either internally load dominated or envelope load dominated, each having a characteristic balance point temperature. |
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==Case Study== |
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Architects and building designers should holistically consider building function and envelope design before construction; however, balance point testing of a building post construction is important to ensure thermal design integrity. Properly understanding and characterizing skin or internal load dominated buildings can help facilitate overall energy consumption savings by properly setting thermostat temperature to avoid excessive heating or cooling. |
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A balance point test study was performed on Kroch Library at Cornell University in Ithaca, New York to better understand how its unique, underground location attributed to the building performance and whether Kroch Library was a skin or internal load dominated building. Hypothesis testing was on the basis that Kroch Library was an internal load dominated building with a low balance point. |
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Internally load dominated buildings have high internal heat gains from occupants, lighting and equipment. These buildings are usually compact with a low surface-area-to-volume ratio and many exterior walls in each room. The high internal heat gains allow the building to not be strongly affected by outdoor conditions. Large office spaces, schools and auditoriums are typical examples of internal load dominated buildings where the balance point temperature is around {{convert|10|C}}. |
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The study investigated three primary modes of energy exchange within the building: |
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<ref name=Lechner /> |
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<br> |
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#Heat transfer through the building envelope was further delineated through three building envelope components (ground heat transfer rate, wall heat transfer rate, roof heat transfer rate). In each component, architectural drawings provided surface area or perimeter dimensions and material compositions to which U or R values could be inferred. |
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#Heat transfer through the building envelope was developed by determining glazing surface area through a relation to the total building surface area and using architectural drawing to extract glazing material and U values. |
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#Heat gains through internal loads were compiled through several components: |
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**{{underline|Lighting Heat Gains}}: Counted total number of light fixtures and determining wattage for each fixture |
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**{{underline|Occupancy Heat Gains}}: Assumed 40% of the occupancy allowance set by the initial architectural plans and selected predetermined activity levels from ASHRAE standards |
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**{{underline|Equipment Heat Gains}}: Counted total number of computers and other office equipment with idling heat released values inferred. |
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The result of the cumulate energy flows determined that Kroch Library is an internal load dominated building <ref name=Walsh>{{cite web |url= http://www.arch.ced.berkeley.edu/vitalsigns/bld/toolkit_studies/Energy%20in%20the%20Balance.pdf |title=Energy in the Balance |last1=Walsh |first1=J. Scott |last2=Jeyifous |first2=Olalekan |website=UC Berkeley |publisher= College of Environmental Design |accessdate=25 November 2014}}</ref> |
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| ⚫ | Envelope load dominated buildings have significant heat loss through the building envelope. These buildings have a high surface-area-to-volume ratio with few exterior walls in each room. Outdoor conditions strongly affect these buildings due to a lack of internal heat gains. Residences, small office buildings and schools are typical examples of skin load dominated buildings where the balance point temperature is set around {{convert|15|C}}.<ref name=Lechner /> |
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==Degree Days== |
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Use of heating degree days (HDD) and cooling degree days (CDD) is the practice of counting the number of days each year for which it is necessary to use energy to heat a building or cool a building. Although degree days are calculated based on recorded energy use in the building, the balance point temperature of the building determines whether a building will annually have more HDD or CDD. A low balance point temperature (relative to the local climate) indicates that the building will be more likely to need additional cooling, while a high balance point temperature indicates that it is more likely to need heating. Ideally, a building should be designed such that the balance point temperature is as near as possible to the average outdoor temperature of the local climate, which will minimize both the CDD and HDD <ref name=Walsh /> |
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| ⚫ | [[Solar gain]]s can hamper internal load dominated buildings, contributing to overheating, while helping skin dominated buildings that lose heat due to poor envelope performance. Therefore, [[architect]]s and [[building design#Building designer|building designers]] must strategically control [[solar gain]]s based on the building characteristics.<ref name=Utzinger /> |
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==Degree days== |
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The concepts of [[degree day]]s and balance point temperature are interconnected. By summing the differences between the balance point temperature and the outdoor temperature over a period of time, the resultant value is degree-time. Use of daily mean temperature data in the summation results in '''degree days''', although degree hours or even degree minutes may be possible depending upon the granularity of the data used. The degree day is often further broken down into [[heating degree day]]s (HDD), in which energy will need to be spent to heat the space, and cooling degree days (CDD), in which the space will need cooling (either through an input of energy or by natural means). This is achieved by counting any positive difference between the balance point temperature and the outdoor air temperature as HDD, and either discarding the remaining data or considering them to be CDD. Although degree days are calculated based on recorded energy use in the building, the balance point temperature of the building determines whether a building will annually have more HDD or CDD. A low balance point temperature (relative to the local climate) indicates that the building will be more likely to need additional cooling, while a high balance point temperature indicates that it is more likely to need heating. Ideally, a building should be designed such that the balance point temperature is as near as possible to the average outdoor temperature of the local climate, which will minimize both the CDD and HDD.<ref name=Walsh>{{cite web|url=http://www.arch.ced.berkeley.edu/vitalsigns/bld/toolkit_studies/Energy%20in%20the%20Balance.pdf |title=Energy in the Balance |last1=Walsh |first1=J. Scott |last2=Jeyifous |first2=Olalekan |website=UC Berkeley |publisher=College of Environmental Design |accessdate=25 November 2014 |url-status=dead |archiveurl=https://web.archive.org/web/20131126044009/http://arch.ced.berkeley.edu/vitalsigns/bld/toolkit_studies/Energy%20in%20the%20Balance.pdf |archivedate=26 November 2013 }}</ref> |
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In order to determine the balance point temperature, either before a building is made or when trying to optimize the building, it is often necessary to create a mathematical model of the situation. The most important part of this for balance point temperature is often internal loads, which do not have a linear relationship to balance point temperature, making modeling a challenge. In the past, semi-parametric systems have been put in place to solve this problem <ref name=Rice>{{cite journal |last=Engle |first=Robert |last2=Granger |first2=C.W.J. |last3=Rice |first3=John |last4=Weiss |first4=Andrew |date=1986 |title=Semiparametric estimates of the relationship between weather and electricity sales |journal=Journal of the American Statistical Association |volume=81 |issue=395 |pages=310-320 |accessdate=26 November 2014}} </ref>. An example of this aould be the work carried out Jeffrey Dubin in 2008. In this, assuming that Equation 2 may be used to approximate the amount of heating energy needed to be put into a space in order for it to remain within the comfort band, then Equation 3 may be used to model the balance point temperature, given varying internal thermal characteristics (insulation levels, thermal loads, etc). |
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<br>Equation 2: ''{{math|Q {{=}} w<sub>0</sub> + w<sub>1</sub> * (t<sub>i</sub> - t<sub>o</sub>) + w<sub>2</sub> * (t<sub>i</sub> - t<sub>o</sub>)<sup>2</sup> }} |
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<br>Equation 3: ''{{math|t<sub>b</sub> {{=}} t<sub>i</sub> + {{sfrac|w<sub>1</sub>|2w<sub>2</sub>}}(1 –(1 * 4w<sub>2</sub>w<sub>1</sub><sup>2</sup>w<sub>0</sub>)<sup>1/2</sup>)}} |
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<br>Where: |
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*''Q'' is heat loss per hour due to indoor-outdoor temperature differential |
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*''w<sub>0</sub>'', ''w<sub>1</sub>'', and ''w<sub>2</sub>'' are functions of the dwelling's thermal characteristics (due to size, insulation, and interior heat load, etc) |
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*''t<sub>i</sub>'' is the interior temperature |
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*''t<sub>o</sub>'' is the outside temperature |
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*''t<sub>b</sub>'' is the balance temperature<ref name=Dubin>{{cite journal |last=Dubin |first=Jeffrey |date=2008 |title=An integrated engineering-econometric analysis of residential balance point temperatures |journal=Energy Economics |volume=30 |issue=5 |pages=2537-2551 |accessdate=26 November 2014}}</ref> |
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==Modeling== |
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Balance point temperature is frequently used in modeling as a base by which to calculate the energy demand of buildings due to various stressors.<ref name=Amato>{{cite journal |last=Amato |first=Anthony |date=2005 |title=Regional energy demand responses to climate change: Methodology and application to the commonwealth of Massachusetts |journal=Climatic Change |volume=71 |issue=1–2 |pages=175–201 |doi=10.1007/s10584-005-5931-2|bibcode=2005ClCh...71..175A |s2cid=153542755 }}</ref><ref name=Santamouris>{{cite journal |last=Santamouris |first=M. |date=1995 |title= On the performance of buildings coupled with earth to air heat exchangers |journal=Solar Energy |volume=54 |issue=6 |pages=375–380 |doi=10.1016/0038-092x(95)00016-k|bibcode=1995SoEn...54..375S }}</ref> This is achieved by calculating HDD or CDD based on the balance point, and extending these results to estimate energy use. A sensitivity analysis can also be conducted based on the effects of changing the balance point temperature, which may demonstrate the effect on a model of altering internal loads or envelope conditions of a building.<ref name=Amato /> |
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==References== |
==References== |
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{{reflist}} |
{{reflist}} |
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[[Category:Heating, |
[[Category:Heating, ventilation, and air conditioning]] |
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[[Category:Temperature]] |
[[Category:Temperature]] |
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Latest revision as of 18:19, 30 May 2024
The building balance point temperature is the outdoor air temperature when the heat gains of the building are equal to the heat losses.[1] Internal heat sources due to electric lighting, mechanical equipment, body heat, and solar radiation may offset the need for additional heating although the outdoor temperature may be below the thermostat set-point temperature.
The building balance point temperature is the base temperature necessary to calculate heating degree day to anticipate the annual energy demand to heat a building. The balance point temperature is a consequence of building design and function rather than outdoor weather conditions.[2]
Mathematical definition
[edit]The balance point temperature is mathematically defined as:
Equation 1: tbalance = tthermostat - QIHG + QSOL/Ubldg
Where:
- tbalance is the balance point outdoor air temperature, given in °C (°F).
- tThermostat is the building thermostat set-point temperature, given in °C (°F).
- QIHG is the internal heat generation rate per unit floor area due to occupancy, electric lighting and mechanical equipment, given in W/m2 (Btu/s/ft2). This internal heat generation is not constant due to variability in occupancy, lighting, and equipment operation schedule but is largely considered constant to a first order approximation.
- QSOL is the building heat gain per unit floor area due to solar radiation, given in W/m2 (Btu/s/ft2). This heat gain is not constant due to solar variability with time of day and year but is largely considered constant to a first order approximation. In winter, it is reasonable to assume QSOL=0.
- Ubldg is the rate of heat transfer across the building envelope per degree temperature difference between outdoor and indoor temperature and per unit floor area, given in W/K/m2 (Btu/s/°F/ft2). This heat transfer can vary due to variations of fresh air ventilation rate but is largely considered constant to a first order approximation.
This equation is simplified by assuming steady state heat transfer between the building and the environment and only provides an approximate building balance point temperature. The 2013 ASHRAE Handbook – Fundamentals, Chapter F18 provides more rigorous methodologies to calculate the heating loads in a nonresidential buildings. The ASHRAE heat balance method, for example, fully delineates the heat transfer through the inner and outer boundaries of the building wall by incorporating radiative (e.g. sun, indoor surfaces), convective (e.g. indoor and outdoor air), and conductive (e.g. inner to outer boundary) modes of heat transfer.[1]
Determination Methods
[edit]
In real-world scenarios, the balance point may be determined in one of two ways. In the energy signature method, a plot is created mapping energy consumption against mean outdoor temperature. The point on the chart at which weather-independent and weather-dependent electricity or gas demand intersect is the balance point temperature. This method only works if large quantities of data on the building energy use are available, preferably on a daily resolution.[3]
In the performance line method multiple plots of energy consumption against heating degree days (HDD) and cooling degree days (CDD) are created, using a range of balance point temperatures to calculate the degree days. Best-fit second-order polynomials of the form y=ax2+bx+c are then applied to the plots, which show various levels of curvature across the range of the data depending on the accuracy of the balance point temperature. In plots with overly high balance point temperatures the a variable is positive, resulting in an upward curve, while plots with low balance point temperatures curve downward due to a negative a variable. The plot in which a is closest to zero represents the most accurate balance point temperature. This method may be applied to buildings in which the availability of energy use data is less granular, perhaps only available on a weekly or monthly basis.[4]
Building characteristics
[edit]A building's thermal characteristics may be described as either internally load dominated or envelope load dominated, each having a characteristic balance point temperature.
Internally load dominated buildings have high internal heat gains from occupants, lighting and equipment. These buildings are usually compact with a low surface-area-to-volume ratio and many exterior walls in each room. The high internal heat gains allow the building to not be strongly affected by outdoor conditions. Large office spaces, schools and auditoriums are typical examples of internal load dominated buildings where the balance point temperature is around 10 °C (50 °F). [2]
Envelope load dominated buildings have significant heat loss through the building envelope. These buildings have a high surface-area-to-volume ratio with few exterior walls in each room. Outdoor conditions strongly affect these buildings due to a lack of internal heat gains. Residences, small office buildings and schools are typical examples of skin load dominated buildings where the balance point temperature is set around 15 °C (59 °F).[2]
Solar gains can hamper internal load dominated buildings, contributing to overheating, while helping skin dominated buildings that lose heat due to poor envelope performance. Therefore, architects and building designers must strategically control solar gains based on the building characteristics.[1]
Degree days
[edit]The concepts of degree days and balance point temperature are interconnected. By summing the differences between the balance point temperature and the outdoor temperature over a period of time, the resultant value is degree-time. Use of daily mean temperature data in the summation results in degree days, although degree hours or even degree minutes may be possible depending upon the granularity of the data used. The degree day is often further broken down into heating degree days (HDD), in which energy will need to be spent to heat the space, and cooling degree days (CDD), in which the space will need cooling (either through an input of energy or by natural means). This is achieved by counting any positive difference between the balance point temperature and the outdoor air temperature as HDD, and either discarding the remaining data or considering them to be CDD. Although degree days are calculated based on recorded energy use in the building, the balance point temperature of the building determines whether a building will annually have more HDD or CDD. A low balance point temperature (relative to the local climate) indicates that the building will be more likely to need additional cooling, while a high balance point temperature indicates that it is more likely to need heating. Ideally, a building should be designed such that the balance point temperature is as near as possible to the average outdoor temperature of the local climate, which will minimize both the CDD and HDD.[5]
Modeling
[edit]Balance point temperature is frequently used in modeling as a base by which to calculate the energy demand of buildings due to various stressors.[6][7] This is achieved by calculating HDD or CDD based on the balance point, and extending these results to estimate energy use. A sensitivity analysis can also be conducted based on the effects of changing the balance point temperature, which may demonstrate the effect on a model of altering internal loads or envelope conditions of a building.[6]
References
[edit]- ^ a b c Utzinger, Michael; Wasley, James. "Vital Signs Curriculum Materials Project" (PDF). UC Berkeley. College of Environmental Design. Archived from the original (PDF) on 12 June 2012. Retrieved 25 November 2014.
- ^ a b c Lechner, Norbert (2009). Heating, Cooling, Lighting: Sustainable Design Methods for Architects. Hoboken, NJ: John Wiley & Sons.
- ^ Lee, Kyoungmi; Baek, Hee-Jeong; Cho, ChunHo (2014). "The Estimation of Base Temperature for Heating and Cooling Degree-Days for South Korea". Journal of Applied Meteorology and Climatology. 53 (2): 300–309. Bibcode:2014JApMC..53..300L. doi:10.1175/jamc-d-13-0220.1.
- ^ Day, A. R.; Knight, I.; Dunn, G.; Gaddas, R. (2003). "Improved methods for evaluating base temperature for use in building energy performance lines". Building Services Engineering Research and Technology. 24 (4): 221–228. doi:10.1191/0143624403bt073oa. S2CID 111019051.
- ^ Walsh, J. Scott; Jeyifous, Olalekan. "Energy in the Balance" (PDF). UC Berkeley. College of Environmental Design. Archived from the original (PDF) on 26 November 2013. Retrieved 25 November 2014.
- ^ a b Amato, Anthony (2005). "Regional energy demand responses to climate change: Methodology and application to the commonwealth of Massachusetts". Climatic Change. 71 (1–2): 175–201. Bibcode:2005ClCh...71..175A. doi:10.1007/s10584-005-5931-2. S2CID 153542755.
- ^ Santamouris, M. (1995). "On the performance of buildings coupled with earth to air heat exchangers". Solar Energy. 54 (6): 375–380. Bibcode:1995SoEn...54..375S. doi:10.1016/0038-092x(95)00016-k.