3. Modeling of DER Devices
3.2 Solar water heating device
3.2.1 Mathematical modeling
The detailed arrangement of the heat pipe evacuated tubular collector is shown in Figure 3-6. The portion of the heat pipe being touched with the fin is the boiler portion. The condenser is a short section in good thermal contact with the pipe or duct through which the fluid will be heated and pumped.
Figure 3-6 Detailed arrangement of heat pipe evacuated tubular collector
Solar radiation is projected on the metallic absorber plate through the enclosure of the evacuated tube, and converted to heat by the absorber plate, making the heat-transfer medium vaporized in the boiler portion of the heat pipe. Once the vapor rises to the condenser section of the heat pipe, the heat is transferred to the fluid in the fluid circulating tube through the heat exchanger block with the vapor itself condensed into liquid and flowing back into the boiler portion by gravity. The above process is repeated circularly, making the temperature of the fluid in the fluid circulating tube increasing ceaselessly. At the same time, the heated absorber plate and the fluid circulating tube dissipate part of the heat to the surrounding environment via various pathways inevitably.
Equivalent thermal network of the heat pipe evacuated tubular collector is shown in Figure 3-7. If all losses are assumed occurring to a common sink temperature, following Energy Conservation Law, rates of actual useful energy gain from the solar can be calculated as
u tu p g p t b p a
Q N A G τ α U U T T (3.9) where
Qu is the rates of addition of energy from solar collector to the load, J/s;
Ntu is the tube number;
Ap is the absorbed plate area of a SWH tube, m2; τg is the transmittance of glass pipe;
αp is the absorptance of absorbed plate;
Ut is evacuated tube heat loss coefficient, W/(m2·K);
Ub is the insulation box heat loss coefficient, W/(m2·K);
Tp is the mean absorbed plate temperature, assumed as a constant value, K.
44 Installed Capacity Optimization in Combination of DER Devices for Residential Buildings
Figure 3-7 Equivalent thermal network of heat pipe evacuated tubular collector
There are radiation and convection between the glass envelope and the ambient environment, but there are only radiation between absorber plate and the glass envelope as we considering no convection and conduction in the vacuum space. Therefore, evaluated tube heat loss coefficient can be expressed as
1 1 1
t p g g sky g sky
r c r
U h h h
(3.10)
where
p g
hr is the radiation heat transfer coefficient between absorber plate and glass envelope, W/(m2·K);
g sky
hc is the convection heat transfer coefficient between glass envelope and background sky, W/(m2·K);
g sky
hr is the radiation heat transfer coefficient between glass envelope and background sky, W/(m2·K).
According to the principle of radiation heat transfer (Holman, 1997; John, Lienhard, John, & Lienhard, 2008), the two radiation heat transfer coefficients can be deduced and expressed as
4 4
2 1 2 1
/ 1 1
p g p
p g
r p g
p g p g g
σ T T A
h T T ε A A ε
(3.11) where
σ is the Stefan-Boltzmann constant which is 5.6697×10-8 W/(m2·K4);
Tg is the glass tube temperature, K;
εp is emissivity of absorbed plate;
Ag is the glass tube surface area, m2;
Ap g is the area between absorber plate and glass envelope, m2; εg is the emissivity of glass tube.
4 4
g g sky
g sky r
g a
ε σ T T
h T T
(3.12)
where, Tsky is the background sky temperature, K.
In accordance with the principle of thermal balance, the heat loss of the evacuated tube can be expressed in another form as
p g
t p a r p g
U T T h T T (3.13)
Collector heat removal factor is defined as a quantity that relates the actual useful energy gain of a collector to the useful gain if the whole collector surface was at the fluid inlet temperature. In equation form it is
'p 1 p t b
R
p t b p
C m A U U F
F exp
A U U C m
(3.14)
where
FR is the collector heat removal factor;
Cp is the water specific heat at constant pressure, which is 4.2×103 J/(kg·K);
m is the mass flow rate of fluid in the fluid circulating tube, kg/s;
F' is the collector efficiency factor, which can be calculated by various parameters of the absorb plate, i.e. collector heat lost coefficient and values of heat transfer coefficient between boiler portion of heat pipe and absorber plate (Duffie &
Beckman, 1991).
46 Installed Capacity Optimization in Combination of DER Devices for Residential Buildings
Therefore, rates of actual useful energy gain from the solar can be rewritten by solving the resulting equations simultaneously, that is to say, Equation 3.9 can be expressed as
g
u t p R p t b i a
Q N A F G τ α U U T T (3.15) where Ti is the fluid inlet temperature, which is equals to the water temperature in water storage tank, K.
Considering about the energy loss from the ducts and pipes leading to and returning from the collector, equation for calculating the useful energy gain of the SWH device can be expressed as the same form as Equation 3.15, only with the value of
τ αg p and
Ut Ub
being corrected, following the research of Beckman (1978) (Duffie &Beckman, 1991).
As shown in Figure 3-8, the heat is transferred by sending hot water from the collector to the water tank, and replenishing the water from the water tank simultaneously, sending the hot water in the water tank ahead of the load, with the municipal water supplied in return. As the water tank involved is thought of an unstratified tank filling with water, the time-dependent temperature of which is assumed to be uniform in this paper, at any instant, the energy balance of the tank yield can be expressed as
, '
, , , ,
s sw
p tank sw u s sw tank s sw s sw a sw
C V dT Q L U A T T
dt (3.16)
where
is thedensity of water, 1 kg/ L;
, tank sw
V is the tank volume of SWH, L;
,
Ts sw is the water temperature in water storage tank of SWH, K;
t is the time instant, s;
,
Ls sw is the rates of removal of energy from SWH to the load, J/s;
UtankAs sw
is the heat loss coefficient-surface area product of water storage tank of SWH, W/K;' ,
Ta sw is the ambient temperature for water storage tank of SWH, K.
Figure 3-8 Water and heat flow path of a typical solar thermal collector with an unstratified water tank
Specially, when the temperature in the tank reaches an upper limit value, the excess heat will be exhausted, that is while dTs sw, dt 0.