Sequence of Analysis; Building Energy Balance; Weather Data

Energy Efficient Buildings

Sequence of Analysis; Building Energy Balance; Weather Data

Building Energy Use in the U.S.

U.S. building energy use is described in figures below. This class will focus on increasing renewable energy use in buildings and on improving the energy efficiency of space heating, space cooling and lighting.

Source: Buildings Energy Data Book, U.S. Department of Energy

Sequence of Analysis

The sequence for designing a building heating and cooling system is shown below.

Our sequence in class:

1. Design space to minimize space loads and energy use
2. Design distribution system to minimize distribution loads and energy use
3. Design primary heating and cooling system to minimize plant energy use

Building Energy Balance

Heat flows into and out of a space are typically called “loads”. To determine the net load on a space, sum the components of the load in an energy balance.

Net Heating Load

During winter, the major sources of heat gain/loss into/from a building are shown below.

The net heat loss out is called the heating load. Based on an energy balance, the net heat load out is:

Qnet,out = (Qwalls + Qwindows +Qceiling + Qdoors + Qinfiltration + Qground)out

– (Qsolar + Qpeople + Qelectricity)in

Heating Energy Use

To maintain the interior of the building at a steady temperature, the furnace must provide enough heat to the space, Qfurnace, to balance the net energy loss from the space, Qnet,out. Because furnaces are not 100% efficient, some of the energy supplied to the furnace, Qnatural gas, is lost in the exhaust, Qexhaust.

Although energy balances can be written in a variety of forms, I strongly recommend writing energy balances in the following form:

Or, on a rate basis:

In this case, an energy balance on the house gives:

Qfurnace – Qnet,out = (dE/dt)house = 0 (if the house temperature remains constant, i.e. Steady State)

It follows that:

Qfurnace = Qnet,out

Efficiency is defined as:

In the case of a furnace, the useful output is Qfurnace and the required input is Qnatural gas. Thus, the efficiency of a furnace is:

η furnace = Qfurnace / Qnatural gas

The efficiencies of furnaces have improved over the years from about 65% to 95%. The efficiency equation can be rearranged to determine the natural gas energy use consumed by the furnace.

Qnatural gas = Qfurnace / η furnace

Example

Consider a building has the following average loads in winter:

Qpeople = 13,000 Btu/h, Qsolar = 3,000 Btu/h, Qelec = 3,000 Btu/h, Qwalls = 20,000 Btu/h, Qwindows = 15,000 Btu/hr,Qdoors=1,500 Btu/hr, Qceiling = 12,500 Btu/hr, Qinfiltration = 15,000 Btu/hr, Qground = 5,000 Btu/hr.

Calculate the Qnet,outof the building.

Qnet,out = (Qwalls + Qwindows + Qceiling + Qdoors + Qinfiltration + Qground)out

– (Qsolar + Qpeople + Qelectricity)in

Qnet,out = (20,000+15,000+12,500+1,500+15,000+5,000)Btu/hr

– (3,000+13,000+3,000) Btu/hr

Qnet,out = 50,000 Btu/hr

The furnace of this buildingoperates 1,000 hours over the winterand is 80% efficient. Natural gas costs $10 /mmBtu. Calculate the cost of fuel for the furnace over a winter.

Qfurnace = Qnet,out =50,000 Btu/hr

Qnatural gas = Qfurnace / η furnace = 50,000 Btu/hr / 80% = 62,500 Btu/hr

Qnatural gas, yr = Qnatural gas x HPY = 62,500 Btu/hr x 1,000 hr/yr = 625,500,000 Btu/yr

Cnatural gas, yr = Qnatural gas,yr x Cng = 625.5 mmBtu/yr x $10 /mmBtu = $6,255 /yr

Net Cooling Load

During summer, the major sources of gain into a building are shown below. Note that ground losses/gains in the summer are typically small and are here assumed to be negligible.

The net heat load into a building is called the cooling load. Based on an energy balance, the net heat load in is:

Qnet,in = (Qsolar + Qpeople + Qelectricity + Qwalls + Qwindows +Qceiling + Qdoors + Qinfiltration)in

Cooling Energy Use

To maintain the interior of the building at a steady temperature, the air conditioner must remove enough heat from the space, Qac, to balance the net energy gain to the space, Qnet,in.

In this case, an energy balance on the house gives:

Qnet,in – Qac = (dE/dt)house = 0 (SS)

It follows that:

Qac = Qnet,in

To pump heat “uphill” from the cool space to the hot outdoors, electric air conditioners require electrical energy Welec. As before, efficiency is defined as:

For an electric air conditioner, the useful output is Qac and the required input is Welec . Thus, the efficiency of the air conditioner is:

η ac = Qac / Welec

The efficiency equation can be rearranged to determine the electricity consumed by an air conditioner.

Welec = Qac / η ac

The efficiency of air conditioners is measured in several ways. The coefficient of performance (COP) is a non-dimensional measure of steady state efficiency at a single set of operating conditions. The COP of typical air conditioners is about 3, which means that an air conditioner removes 3 units of heat from a space for every unit of electrical work it consumes. Air conditioner efficiency varies with the temperature of the air returned to the air conditioner and the temperature of the outdoor air to which heat is rejected. The average efficiency rating of air conditioners over a season is reported as the Seasonal Energy Efficiency Rating (SEER) with units Btu/Wh. The electricity consumed by an air conditioner can be calculated using either COP or SEER with proper attention to units.

Welec= Qac/ η ac= Qnet,in/ COPac= Qnet,in / SEER

Example

Consider a building has the following average loads in summer:

Qpeople = 3,500 Btu/h, Qsolar = 11,000 Btu/h, Qelec = 7,500 Btu/h, Qwalls = 2,500 Btu/h, Qwindows = 3,000 Btu/hr,Qdoors=750 Btu/hr, Qceiling = 750 Btu/hr, Qinfiltration = 3,000 Btu/hr.

Calculate the Qnet,in of the building.

Qnet,in = (Qsolar + Qpeople + Qelectricity + Qwalls + Qwindows + Qceiling + Qdoors + Qinfiltration)in

Qnet,in = (11,000+3,500+7,500+2,500+3,000+750+750+3,000) Btu/hr

Qnet,in =32,000 Btu/hr

The air conditioner for this buildingoperates 1,000 hours over the summer, with a COP of 3.2. Electricity costs $0.10 /kWh. Calculate the cost of electricity for the air conditioner over the summer.

Qac = Qnet,in = 32,000 Btu/hr

Welec = Qac / COP = (32,000 Btu/hr / 3.20) / 3,412 Btu/kWh= 2.93 kW

Welec, yr = Welec x HPY = 2.93 kW x 1,000 hr/yr = 2,930 kWh/yr

Celec, yr = Welec,yr x Celec = 2,930 kWh/yr x $0.10 /kWh = $293 /yr

Example

Calculate the cost of operating an air conditioner over a summer if the average net cooling load is 36,000 Btu/hr, the SEER is 12 Btu/Wh, the air conditioner operates 1,000 hours over the summer, and electricity costs $0.10 /kWh.

Qac = Qnet,in = 36,000 Btu/hr

Welec = Qac / SEER = (36,000 Btu/hr / 12 Btu/Wh) = 3.00 kW

Welec, yr = Welec x HPY = 3.00 kW x 1,000 hr/yr = 3,000 kWh/yr

Celec, yr = Welec,yr x Celec = 3,000 kWh/yr x $0.10 /kWh = $300 /yr

Weather Data

The net heat gain or loss to buildings depends on weather. The three most important weather variables are:

• Outdoor air dry-bulb temperature
• Outdoor air humidity
• Solar radiation

Three types of weather data are commonly used for building heating and air conditioning design and analysis:

• Typical weather (for estimating typical energy use or savings)
• Actual weather (for calibrating energy use calculated by a theoretical model to measured energy use)
• Design weather (for calculating maximum expected building loads to size heating and air conditioning equipment)

Typical Weather Data (TMY2, TMY3 and EPW files)

To determine typical weather conditions, meteorologists analyzed 30 years (1961- 1990) of hourly data from 239 U.S. weather stations. Based on this data, meteorologists created “Typical Meteorological Year”, TMY, files for U.S. locations. The second generations of these files are called TMY2 files. The third generation of these files are called TMY3 or TM3 files, and are derived from data from 1991-2005. In this class, we will use TMY3 files. TMY3 sites are noted as Class I, Class II or Class III, with the most robust data from Class I sites.

TMY3 files use commas to separate the fields (i.e. they are “comma delimited”), and carry the field name extenstion *.csv. The *.csv files can be read directly into Microsoft Excel. After input into Microsoft Excel, the two header lines and first eight fields of the first three data records of the TMY3 file for Dayton, Ohio are shown below. The first header line indicates site number, site name, site state, timezone, longitude, latitude and elevation (m). The second header line indicates the name and units of each data field. The file then includes 8,760 hourly records (rows) of data, with each record containing 68 fields (columns). TMY3 files use SI units.

TMY3 files for 1,020 U.S. locations, and a manual which describes the data, can be downloaded from the National Renewable Energy Laboratory at:

Many building energy simulation programs use these files as input, since these files represent the typical meteorological conditions for a given site and contain data on a short enough time interval to quantify many transient effects. Because they represent typical rather than extreme conditions, they are not suited for designing systems to meet the worst-case conditions occurring at a location.

The U.S. Department of Energy is developing a new building energy simulation program called Energy Plus. The weather files for Energy Plus are called Energy Plus Weather, EPW, files. EPW files contain the same data as TMY2 files, with the advantage of being available for many international locations. EPW files, and a manual which describes the data, can be downloaded from:

WeaTran Weather Data Translator

The weather translator software, WeaTran, can read TMY2, TMY3and EPW files and create smaller simpler files that still contain the essential solar, temperature and humidity data. The output files from WeaTran are easily loaded into spreadsheets for further analysis. In addition, Weatran can calculate solar radiation on up to four vertical exposures from data in the input file. WeaTran reads hourly TMY2, TMY3 or EPW files and makes two types of output files. The first type contains time series data and the second type contains “bin” data. WeaTran can be downloaded from a link on the class homepage.

WeaTran Time Series Data

To produce time series output files, WeaTrantranslates hourly TMY2, TMY3 or EPW files into either hourly, daily, or monthly time intervals, depending on user specification. In addition, WeaTran also translates hourly TMY2, TMY3 or EPW files into day-type output, which includes one representative day of hourly data from each month. Data output files are named according to the following convention:

• First set of characters are the basename of TMY or EPW file
• Second two characters are either “HR”, “DY”, “MO” or “DT” corresponding to whether the output file contains hourly, daily, monthly or day-type data.
• Last two characters are either “US” or “SI” depending on whether the output data are reported in US or SI unts
• The filename extension is .TXT

If US units are specified, in each data file:

Ta(F) = average air dry-bulb temperature

Sol-H(Btu/ft2dy) = total solar radiation on a horizontal surface

Sol-E(Btu/ft2dy) = total solar radiation on a vertical east-facing surface

Sol-S(Btu/ft2dy) = total solar radiation on a vertical south-facing surface

Sol-W(Btu/ft2dy) = total solar radiation on a vertical west-facing surface

Sol-N(Btu/ft2dy) = total solar radiation on a vertical north-facing surface

w(lbw/lba) = average air specific humidity

Tg(F) = average effective ground temperature

For example, the first 10 records from the hourly time series output file DaytonOH_HR_US.TXT and U.S. units from input file DaytonOH. TMY3 are shown below.

The first 10records from the daily time series output file DaytonOH_DY_US.TXT and U.S. units from input file DaytonOH. TMY3are shown below.

The 12records from the monthly time series output file DaytonOH_MO_US.TXT and U.S. units from input file DaytonOH.TMY3 are shown below.

The first 10 records from the day-type output file DaytonOH_DT_US.TXT and U.S. units from input file DaytonOH.TMY3 are shown below. To create day-type files, WeaTran selects the single day in each month with the average temperature closest to the monthly average temperature, and prints all 24 hours for that day. Thus, day-type files contain a header line plus 12 x 24 = 288 records.

Time series output files from WeaTran can be loaded into Excel by opening the text file, and specifying that the fields are ‘delimited’ by ‘spaces’.

WeaTran Bin Data

WeaTran also translates hourly TMY2, TMY3 or EPW files into temperature bin data. A bin data file created from input file DaytonOH.TMY3 is shown below.

WeaTran Heating and Cooling Degree Day Data

WeaTran also translates hourly TMY2, TMY3 or EPW files into heating and cooling degree day data. The first 10 lines of heating and cooling degree day data created from input file DaytonOH.TMY3 are shown below. Tb (F) is the reference temperature from which the degree days (F-day/year) are calculated.

Actual Weather Data

To improve the accuracy of theoretical models of building energy use, it is useful to compare the predicted energy use of the theoretical model to measured energy use of the building. The process of comparing the predicted energy use from theoretical models to measured energy use, and adjusting the theoretical model to improve the fit is called “calibrating the model”.

Actual Hourly Temperature and Humidity Data

The best source of weather data for calibrating models is measured hourly temperature, humidity and solar radiation data from the period during which the building energy use was measured. Unfortunately, however, it is often difficult to obtain measured hourly temperature, humidity and solar radiation data over specific periods. However, some actual hourly temperature, humidity and windspeed data are available at no charge from the Energy Plus web site at:

NASA also operates posts actual weather and solar data at:

Actual Average Daily Temperature Data

In contrast to hourly data, average daily temperature data are widely available. For example, average daily temperature data from 1995 to present for over 300 US and international sites are available for no charge from:

The first 10 records of the average daily temperature file for Dayton, OH is shown below.

Synthesized Hourly Temperature, Humidity and Solar Data

Because outdoor air temperature is typically the single most important weather variable influencing building energy use, hourly weather data files synthesized from average daily temperature data are often the best weather data available for calibrating models to measured energy use. The building energy simulation program, ESim, can synthesize hourly weather data from daily temperatures using solar geometry and the typical relations between weather variables. More information about synthesizing “near actual” hourly weather data from daily temperature data is available in the ESim help menu. ESim can be downloaded from the class homepage.

Design Weather Data

Design weather data sets include the hottest and coldest expected weather conditions for a location. This information is used to size heating and cooling systems so that they can handle the largest expected loads. Design weather data are available in the ASHRAE Fundamentals Handbook for 4,422 sites around the world. The full set of ASHRAE design data for Dayton Ohio, USA are shown below.

Design Heating and Cooling Temperatures

For heating, ASHRAE determined the “99.6%” and “99.0%” design conditions. This means that the actual hourly temperatures were greater (warmer) than the design temperature 99.6% or 99.0% of all annual hours. The peak heating load design temperatures for Dayton, OH are shown below. To ensure that the heating system is large enough to handle the coldest expected temperatures, use the 99.6% design temperature.

For Dayton / DB
99.6% / -1 °F
99% / 5 °F

For cooling, ASHRAE determined “0.4%” and “1.0%” design conditions for temperature (Tdb) and humidity (Twb & Tdp), such that the actual hourly temperatures were greater (warmer) than the design temperatures 0.4% or 1.0% of all annual hours. In addition, ASHRAE determined the mean coincident wet bulb temperature (MCWB) for each design condition, which is the mean wet bulb temperature at the specified dry bulb temperature. MCWB temperature is used for calculating latent cooling loads. The peak cooling load design temperatures for Dayton, OH are shown below. To ensure that the cooling system is large enough to handle the warmest expected conditions, use the 0.4% design temperatures.

For Dayton / DB / MCWB
0.4% / 90 °F / 74 °F
1.0% / 88 °F / 73 °F

Design Solar Radiation

ASHRAE recommends the following method for calculating maximum values of solar radiation on a surface, Et. Following this method, the total radiation on a surface, Et, is the sum of the direct radiation ED, the diffuse radiation Ed and the radiation reflected from the ground Er. The method uses local standard time, LST, the local standard meridian, LSM, the local longitude, LON, the angle of the surface from south and the tilt angle of the surface from horizontal as inputs. The method is demonstrated in the example below.