Page 560 - Mechanical Engineers' Handbook (Volume 4)

6 Computer Programs 549

yearly HVAC system energy consumption, degree day and bin methods, can provide only a
general estimate based on the averaged weather data. Their use is limited to first-order energy
consumption estimates. The introduction of computer methods in the early 1960s enabled
the development of several different simulation methodologies and over a dozen of energy
simulation programs.
Energy simulation programs use two different methods for the calculation of heating/
cooling loads: the weighting factor method and the energy balance method. The weighting
factor method is older and simpler than the energy balance method. Based on building
materials, weighting factors are precalculated for different building components. The weight-
ing factors express the convective gains over the total heat gain incoming on a building
element in a time sequence. The data based off of the weighting factor for different elements
are used to calculate the cooling loads. The energy simulation programs DOE-1 and DOE-
2 are the most popular representatives of this simulation technology.
The energy balance method is currently used by the most popular energy simulation
programs such as EnergyPlus and ESP-r. The energy balance method uses a heat balance
for each zone, typically a single space, in a building. Each building zone surface has con-
ductive, convective, and radiative heat fluxes in balance. All of the energy balance equations
are put together in a matrix to resolve all of the zone fluxes simultaneously. The energy
balance equation for the zone air is
˙ qA Q Q Q Q Q V room p (26)
˙
N
c T
˙
˙
˙
˙
˙
i 1 i,c i lights people appliances infiltration heat extraction t
where N i 1 ˙ qA i is the convective heat transfer from enclosure surfaces to room air, N is
i,c
˙
˙
˙
the number of enclosure surfaces, A is the area of surface i, Q lights , Q people , Q appliances , and
i
˙
Q infiltration are the cooling loads of lights, people, appliances, and infiltration, respectively,
˙
˙
Q heat extraction is the heat extraction via HVAC device (equal to cooling loads), V room p / t
c T
is the room air energy change, is the air density, V ˙ room is the room volume flow rate, c is
p
the air specific heat, T is the temperature change of room air, and t is the sampling time
interval.
The energy balance method results not only in the energy requirement data to heat and
cool a building hour by hour, but also in the surface temperature distribution and an average
air temperature needed to evaluating the thermal comfort in a building zone.
6.2 Airflow Simulation Programs
The first generation of airflow simulation programs for buildings were multizone models
used to estimate the building infiltration rates for use in energy simulation programs. Mul-
tizone models define a zone in a building that is connected to other zones by flow paths
both among zones and to the outside environment. In each zone (room), perfect mixing is
assumed with uniform density, temperature, velocities, and species concentration. Mass bal-
ance and energy conservation are applied to each zone
n
dM i
Mass balance: m m source m sin k (27)
ij
dt j 1
n
dQ
Energy conservation: i q q source q sin k (28)
ij
dt j 1
where M is the mass in zone i; Q is the thermal energy in zone i; n j 1 m is the sum of
i
i
ij
mass transported to zone i from other zones, i j; n j 1 q is the sum of thermal energy
ij