11. Annual Energy Consumption

In the thermal analysis of buildings, two quantities of major interest are (1) the size or capacity of the heating and the cooling system and (2) the annual energy consumption. The size of a heating or cooling system is based on the most demanding situations under the anticipated worst weather conditions, whereas the average annual energy consumption is based on average usage situations under average weather conditions. Therefore, the calculation procedure of annual energy usage is quite different than that of design heating or cooling loads.

An analysis of annual energy consumption and cost usually accompanies the design heat load calculations and plays an important role in the selection of a heating or cooling system. Often a choice must be made among several systems that have the same capacity but different efficiencies and initial costs. More efficient systems usually consume less energy and money per year, but they cost more to purchase and install. The purchase of a more efficient but more expensive heating or cooling system can be economically justified only if the system saves more in the long run from energy costs than its initial cost differential.

The impact on the environment may also be an important consideration on the selection process: A system that consumes less fuel pollutes the environment less, and thus reduces all the adverse effects associated with environmental pollution. But it is difficult to quantify the environmental impact in an economic analysis unless a price is put on it.

One way of reducing the initial and operating costs of a heating or cooling system is to compromise the thermal comfort of occupants. This option should be avoided, however, since a small loss in employee productivity due to thermal discomfort can easily offset any potential gains from reduced energy use. The U.S. Department of Energy periodically conducts comprehensive energy surveys to determine the energy usage in residential as well as nonresidential buildings and the industrial sector. Two 1983 reports (DOE/EIA-0246 and DOE/EIA-0318) indicate that the national average natural gas usage of all commercial buildings in the United States is 70,000 Btu/ft² · year, which is worth about $0.50/ft² or $5/m² per year. The reports also indicate that the average annual electricity consumption of commercial buildings due to airconditioning is about 12 kWh/ft² · year, which is worth about $1/ft² or $10/m² per year. Therefore, the average cost of heating and cooling of commercial buildings is about $15/m² per year. This corresponds to $300/year for a 20 m² floor space, which is large enough for an average office worker. But noting that the average salary and benefits of a worker are no less than $30,000 a year, it appears that the heating and cooling cost of a commercial building constitutes about 1 percent of the total cost (Fig. 63). Therefore, even a 1 percent loss in productivity due to thermal discomfort may cost the business owner more than the entire cost of energy. Likewise, the loss of business in retail stores due to unpleasant thermal conditions will cost the store owner many times what he or she is saving from energy. Thus, the message to the HVAC engineer is clear: in the design of heating and cooling systems of commercial buildings, treat the thermal comfort conditions as design constraints rather than as variables. The cost of energy is a very small fraction of the goods and services produced, and thus, do not incorporate any energy conservation measures that may result in a loss of productivity or loss of revenues.

When trying to minimize annual energy consumption, it is helpful to have a general idea about where most energy is used. A breakdown of energy usage in residential and commercial buildings is given in Fig. 64. Note that space heating accounts for most energy usage in all buildings, followed by water heating in residential buildings and lighting in commercial buildings. Therefore, any conservation measure dealing with them will have the greatest impact.

For existing buildings, the amount and cost of energy (fuel or electricity) used for heating and cooling of a building can be determined by simply analyzing the utility bills for a typical year. For example, if a house uses natural gas for space and water heating, the natural gas consumption for space heating can be determined by estimating the average monthly usage for water heating from summer bills, multiplying it by 12 to estimate the yearly usage, and subtracting it from the total annual natural gas usage. Likewise, the annual electricity usage and cost for air-conditioning can be determined by simply evaluating the excess electricity usage during the cooling months and adding them up. If the bills examined are not for a typical year, corrections can be made by comparing the weather data for that year to the average weather data.

For buildings that are at the design or construction stage, the evaluation of annual energy consumption involves the determination of (1) the space load for heating or cooling due to heat transfer through the building envelope and infiltration, (2) the efficiency of the furnace where the fuel is burned or the COP of cooling or heat pump systems, and (3) the parasitic energy consumed by the distribution system (pumps or fans) and the energy lost or gained from the pipes or ducts (Fig. 65). The determination of the space load is similar to the determination of the peak load, except the average conditions are used for the weather instead of design conditions. The space heat load is usually based on the average temperature difference between the indoors and the outdoors, but internal heat gains and solar effects must also be considered for better accuracy. Very accurate results can be obtained by using hourly data for a whole year and by making a computer simulation using one of the commercial building energy analysis software packages.

The simplest and most intuitive way of estimating the annual energy consumption of a building is the degree-day (or degree-hour) method, which is a steady-state approach. It is based on constant indoor conditions during the heating or cooling season and assumes the efficiency of the heating or cooling equipment is not affected by the variation of outdoor temperature. These conditions will be closely approximated if all the thermostats in a building are set at the same temperature at the beginning of a heating or cooling season and are never changed, and a seasonal average efficiency is used (rather than the full-load or design efficiency) for the furnaces or coolers. 

You may think that anytime the outdoor temperature T_o drops below the indoor temperature T_i at which the thermostat is set, the heater will turn on to make up for the heat losses to the outside. However, the internal heat generated by people, lights, and appliances in occupied buildings as well as the heat gain from the sun during the day, Q_gain, will be sufficient to compensate for the heat losses from the building until the outdoor temperature drops below a certain value. The outdoor temperature above which no heating is required is called the balance point temperature T_balance (or the base temperature) and is determined from (Fig. 66)

where Koverall is the overall heat transfer coefficient of the building in W/ºC or Btu/h · ºF. There is considerable uncertainty associated with the determination of the balance point temperature, but based on the observations of typical buildings, it is usually taken to be 18ºC in Europe and 65ºF (18.3ºC) in the United States for convenience. The rate of energy consumption of the heating system is

where η_heating is the efficiency of the heating system, which is equal to 1.0 for electric resistance heating systems, COP for the heat pumps, and combustion efficiency (about 0.6 to 0.95) for furnaces. If K_overall, T_balance, and η_heating, are taken to be constants, the annual energy consumption for heating can be determined by integration (or by summation over daily or hourly averages) as

where DD_heating is the heating degree-days. The + sign above the parenthesis indicates that only positive values are to be counted, and the temperature difference is to be taken to be zero when T_o > T_balance. The number of degreedays for a heating season is determined from

where To, avg, day is the average outdoor temperature for each day (without considering temperatures above Tbalance), and the summation is performed daily

(Fig. 67). Similarly, we can also define heating degree-hours by using hourly average outdoor temperatures and performing the summation hourly. Note that the number of degree-hours is equal to 24 times the number of degree-days. Heating degree-days for each month and the yearly total for a balance point temperature of 65ºF are given in Table 5 for several cities. Cooling degree-days are defined in the same manner to evaluate the annual energy consumption for cooling, using the same balance point temperature.

Expressing the design energy consumption of a building for heating as Q_design = K_overall(T_i – T_o)_design/η_heating and comparing it to the annual energy consumption gives the following relation between energy consumption at designed conditions and the annual energy consumption (Table 22),

where (T_i – T_o)_design is the design indoor–outdoor temperature difference.

Despite its simplicity, remarkably accurate results can be obtained with the degree-day method for most houses and single-zone buildings using a hand calculator. Besides, the degree-days characterize the severity of the weather at a location accurately, and the degree-day method serves as a valuable tool for gaining an intuitive understanding of annual energy consumption. But when the efficiency of the HVAC equipment changes considerably with the outdoor temperature, or the balance-point temperature varies significantly with time, it may be necessary to consider several bands (or “bins”) of outdoor temperatures and to determine the energy consumption for each band using the equipment efficiency for those outdoor temperatures and the number of hours those temperatures are in effect. Then the annual energy consumption is obtained by adding the results of all bands. This modified degree-day approach is known as the bin method, and the calculations can still be performed using a hand calculator.

The steady-state methods become too crude and unreliable for buildings that experience large daily fluctuations, such as a typical, well-lit, crowded office building that is open Monday through Friday from 8 AM to 5 PM. This is especially the case when the building is equipped with programmable thermostats that utilize night setback to conserve energy. Also, the efficiency of a heat pump varies considerably with the outdoor temperatures, and the efficiencies of boilers and chillers are lower at part load. Further, the internal heat gain and necessary ventilation rate of commercial buildings vary greatly with occupancy. In such cases, it may be necessary to use a dynamic method such as the transfer function method to predict the annual energy consumption accurately. Such dynamic methods are based on performing hourly calculations for the entire year and adding the results. Obviously they require the use of a computer with a well-developed and hopefully user-friendly program. Very accurate results can be obtained with dynamic methods since they consider the hourly variation of indoor and outdoor conditions as well as the solar radiation, the thermal inertia of the building, the variation of the heat loss coefficient of the building, and the variation of equipment efficiency with outdoor temperatures. Even when a dynamic method is used to determine the annual energy consumption, the simple degree-day method can still be used as a check to ensure that the results obtained are in the proper range.

Some simple practices can result in significant energy savings in residential buildings while causing minimal discomfort. The annual energy consumption can be reduced by up to 50 percent by setting the thermostat back in winter and up in summer, and setting it back further at nights (Table 23). Reducing the thermostat setting in winter by 4ºF (2.2ºC) alone can save 12 to 18 percent; setting the thermostat back by 10ºF (5.6ºC) alone for 8 h on winter nights can save 7 to 13 percent. Setting the thermostat up in summer by 4ºF (2.2ºC) can reduce the energy consumption of residential cooling units by 18 to 32 percent. Cooling energy consumption can be reduced by up to 25 percent by sunscreening and by up to 9 percent by attic ventilation.