As the important facilities of the energy efficiency buildings, the heat pump
air-conditioning has aroused people's wide concern. The use of simulation technology
in the field of heat pump has played a very important role in operating characteristics
and product innovation of heat pump air-conditioning system (Yan
et al., 2006; Chen et al., 2010). Air
source heat pump system (Zhang et al., 2007),
ground source heat pump system (Xing and Yu, 2009)
and water source heat pump system were studied in detail by simulation technology
(Tang and Ding, 2011). However, these studies have
ignored the hourly load of building itself and simulation is only under ideal
conditions on the simulation of a single air-conditioning equipment, there are
a lot of limitations.
This study focuses on the principle of building loads and the calculation of
air source heat pumpsystem. During the calculation process, the building is
divided into several hot zones. Besides, dozens of air nodes were attributed
to the hot zones in order to establish the building load calculation model.
Meanwhile, the air source heat pump module is established by the air source
heat pump system, air source heat pump units, auxiliary heat source and a simple
building thermal calculation module. Finally, an example shows that the system
is reliable and accurate.
Building load calculation principles: In the simulate calculation, the
building is divided into several hot zones, the air of each heat zone is set
to an air node. The specific calculation model is as follows:
where, Qsurf,i is the convective heat transfer between of air and
the construction of the respective inner surface, W; Qinf,i is the
convective heat transfer due to the penetration of ventilation brought by the
building, W; Qvent,i is the convective heat transfer of the building
hvac system ventilation, W; Qg,c,i is the convective heat transfer
of internal heat source (human, equipment), W; Qcplg,i is the convective
heat transfer by ventilation of the adjacent areas, W.
where, Qr,wi is radiation heat gained from temperature node of wall
surface, W; Qg,r,I,wi is radiation heat transfer between the inner
source heat due to walls, W; Qsol,wi is the amount of solar radiation
absorbed by walls through the window, W; Qlong,wi is long-wave radiation
between the building envelope, W; Qwall,gain is the wall of the internal
heat source, W.
||Heat transfer model collection of building envelope:
where, Ss,i is the amount of radiation absorbed by the inner surface,
W; Ss,o is the amount of radiation of outer surface, W; qr,s,t
is the amount of net radiation in the other face of thermal area, W; qr,s,o
is the amount of net radiation out of the each face of thermal area, W; qs,i
is the convection heat from the inner surface, W.
The time series of surface temperature and heat flow equation is calculated
based on the time step. Superscript k refers to the time series. For previously,
k = 1, etc.
||Windows heat transfer model: The windows are generally
considered as the exterior wall of the heat capacity and the solar radiation
can penetrate but the long-wave radiation cannot. In this study, the windows
were considered to be a two-site model shown in Fig. 1
||Building shield calculation model: Building azimuth is defined
by sloped inclination and azimuth. Relative to the positioning of the building,
the vertical shield face shown in Fig. 2a is through face
angle and elevation angle to define. An angle on spherical coordinate system
for tilt is needed to define for the sloped shield face shown in Fig.
2b to locate the building
||Model of two-site window
The shading coefficient of the building is revised by calculating the proportion
of shield when light incident on the coordinates.
||Effective sky temperature calculation model: The effective
sky temperature used to calculate the long-wave radiation heat transfer
of the building facade with the surrounding environment. The effective sky
temperature is calculated as follows:
where, Ccover is sky transparency, [0-1]; Edif is horizontal
plane scattered radiation, kJ h-1 m-2; EDir
is horizontal plane direct radiation, kJ h-1 m-2; Etotal
is total horizontal radiation; G: acceleration of gravity, m sec-2;
H: altitude, m; Patm is atmospheric pressure, atm; P0
is the atmospheric pressure at the height of h0, atm; ρ0
is the atmospheric density at the height of h0, kg m-3;
ε0 is emissivity in sunny, [0-1]; Tamb is ambient
temperature, °C; Tsat is the dew point temperature of the environment,
°C; Tsky is sky temperature, °C.
||Face angel and elevation angle of vertical barrier and sloped
barrier (a) Face angel and (b) Elevation angle
Air source heat pump system mechanism
||Air source heat pump units: The part of the cold and
heat source is air source heat pump units. It provides cold and heat source
for the entire system and it is also the heart of the whole system. The
main factors affecting its refrigeration heating capacity and power consumption
for a given air source heat pump units are evaporation temperature and condensing
temperature. In the manual of thermal unit product, usually given the cooling
heat capacity and the corresponding power consumption corresponding to the
units according to the inlet water and air temperature and flow of different
source side and user side. Based on the inlet temperature, inlet air temperature
and flow, the amount of cooling (heating) and energy consumption could be
gained by adopting the linear interpolation method. Then the temperature
on both sides of the heat pump units source side, heat transfer and COP
hourly data will be calculated. In heating conditions:
where, Cap is heating capacity; P is power; Qabsorbed is heat pump
heat gain from the source side; Cp is specific heat.
|| Auxiliary heat source heat exchanger model
||Auxiliary heat source: Auxiliary heat source provides
heat to the heat pump source side, the air is heated to a fixed temperature
and then enters to the unit to prevent the source side of the unit frosts
when the source-side temperature is low. As shown in Fig.
3, the strategy of operation of the auxiliary heat source is that the
source side of the auxiliary heat source is not turned on when entering
the unit source-side air temperature is higher than the set value. Open
the auxiliary heat source, the auxiliary heat source works in the power
settings when the source-side temperature is below the set value
The formulations of heat exchanger model are as follows:
where, hout is outlet air enthalpy, kJ kg-1; hin
is inlet air enthalpy, kJ kg-1; hout is outlet air enthalpy,
kJ kg-1; hin is inlet air enthalpy, kJ kg-1.
m: air flow, kg h-1; qη is the efficiency of auxiliary
heat source, kg h-1; UA is heat loss of auxiliary heat source specific
heat, kJ kg-1; is the auxiliary heat nosocomial average air temperature,
°C; Tenv is ambient temperature, °C.
||Simple building thermal calculation module: For the
feedback temperature of building indoor in air source heat pump system,
the building thermal calculation module is set. The module considers building
as the lumped parameter method, the equation of the interior architecture
temperature and humidity changes is as follows:
where, U is the overall heat loss of the building, kJ-1 h-1xm-2xc-1;
Cap is building hot melt, kJ c;-1 Cpair is specific heat
of air, kJ-1 kg-1xc-1; ρair
is air density, kg m-3; Area is the outer area of the building, m2;
Vol is construction cascade, m3; Tvent is mechanical ventilation
temperature, °C; ωvent is mechanical ventilation and humidity;
mvent is the amount of mechanical ventilation, kg h-1;
Tamb is outdoor air temperature, °C; ωamb is
outdoor air humidity, °C; minf is penetrate the ventilation rate,
kg h-1; Qlights is lighting heat gain: kJ h-1;
Qequip is the heat gain of the device, kJ h-1; Qpeop
is the sensible heat of the person in the room, kJ h-1; ωgain
is the obtained wet in a given indoor, kg h-1; Tzone is
indoor temperature, °C; ωZone is indoor humidity, °C;
Qinf is the ventilation heat of penetration, kJ h-1; Qvents
is sensible heat of mechanical ventilation, kJ h-1; Qventl
is latent heat of mechanical ventilation, kJ h-1.
Previous researches on the building heat pump systems do not consider the hourly
load. Therefore, this section designs a rational simulation system with building
hourly load calculation principles and air-source heat pump system operating
Simulation design of building hourly load: Buildings, as for the objects
of heat pump system, create building hourly cooling and heating load calculation
platform in TRNSYS according to the theoretical basis. This calculation platform
focuses on the external structure heat transfer model. The model can gives full
consideration to the periphery structure of the surface heat transfer coefficient,
the activities of the person in the room, lighting, heating equipment, ventilation
on the indoor environment. Then the human thermal comfort evaluation index PMV
PPD can be transmitted online. It can not only reflect the heating load change
of the building itself but also revise shading coefficient to proposed a simple,
scientific and correction method.
Simulation design of air source heat pump system: Build a variety of
heat pump systems computing model by choosing unit types on the basis of calculating
the building load. According to the characteristics of the heat pump air conditioning
system, the hvac system is abstracted into the flow chart shown in Fig.
4, three important content is included: The number of the water pump and
heat pump equipment is adjusted according to load; heat pump host gets infinitely
variable control according to part load rate, the water pump works in the condition
of fixed frequency and variable frequency.
Building hourly load calculation: This study selected a 18 floors office
building, the construction parameters shown in Table 1.
|| Flow chart of heat pump system
|| Main interface of building hourly load calculation software
||Parameter input and calculation results of building hourly
load calculation software, (a) Parameter input and (b) Calculation results
The main interface of building hourly load calculation software is shown in
Fig. 5. The parameter input of the model is presented in Fig.
6a and the calculation results shown in Fig. 6b.
Air source heat pump system calculation: This simulation example heating
peak load is 1078.9 kW and cooling peak load is 1760 kW.There are 4 heat pump
host units and the cold pump and heat pump are one-to-one correspondence. The
cooling capacity of the heat pump host is 450 kW. The single pump flow is 75750
kg h-1 in summer and 46500 kg h-1 in winter. The pump
head is 25 m, the total pump efficiency is 0.60 and pump motor efficiency is
0.90. Auxiliary heating heat source is 20 kW, open the source when the air temperature
is lower than 3°C. There are 150 fan coils, each of Fan coil is 50 W with
efficiency of 0.80, including 6 empty sets, each of set is 3.0 kW with efficiency
of 0.80. This Air-source heat pump system also consists of 6 newly-built Fans.
Besides, the number of Exhaust Fan in this system is 6. The power of each Fan
and Exhaust Fan is 3.7 kW and the efficiency is 0.80.
|| Relevant parameters and description of typical construction
Source and load side are fixed-frequency control, load side water temperature
in winter is 50 and 7°C in the summer.
The main interface of air source heat pump system software is shown in Fig.
7. The parameter settings of air source heat pump and turbine are shown
in Fig. 8a and b, respectively and other
parameters are shown in Fig. 8c.
|| Main interface of air source heat pump system software
||Parameter settings figure of air source heat pump, turbine
and others, (a) Parameter settings figure of air source heat pump, (b) Parameter
settings figure of turbine and (c) Parameter settings figure of others
|| Chart of device power consumption
The output parameters of calculation results include the power consumption
of the device, the host COP, as well as start and stop signals, the source-side
and load-side temperature and the number of device turned on, as shown in Fig.
|| Diagram of Host COP and start and stop signals
|| Figure of source side and load-side temperature
|| Figure of the number of turned on units
It can draw the following conclusions based on the above analysis:
||In this study, the principles and algorithms of the building
load and air source heat pump system are studied and build the building
load calculation model and air source heat pump system model
||On the basis of TRESYS software, the building hourly load simulation system
is established. Three contents is included: the number of the water pump
and heat pump equipment is adjusted according to load; heat pump host gets
infinitely variable control according to part load rate, the water pump
works in the condition of fixed frequency and variable frequency. And on
this basis, the dynamic simulation system of air source heat pump system
based on building hourly load is established
||The example this study shows that the design of the simulation system
is accurate and reliable