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Effect of the mass flow on efficiency for solar thermal geothermic
L. Maifi 1*, T. Kerbache 1 and O. Hioual 2†
1 Physical Chemistry of semi-conductor Laboratory University of Constantine, Algeria.
2 Faculty of Sciences & Technology University of khenchela, Algeria
Abstract - In This paper, a theoretical study of a hybrid renewable energy system that uses geothermal and solar heat sources for water heating, space heating, and space cooling in a residential building in Algeria is presented. The system consists of a geothermal heat pump for heating and cooling, solar collectors for hot water, heat exchanger, turbo compressor and incidental facilities. Based on energy balance, the heat transfer modes of all main components in this system cascade into an order four matrix are calculated by Gauss-Saidel method. The results show state, the total efficiency, electrical and thermal efficiency of this system increase with increasing of the water mass flow rate, and decreases with the increasing of the exchanger cannel depth.
Keywords: Hybrid renewable, Energy system, Water, Heat exchanger, Efficiency, Mass flow, Geothermal
1. INTRODUCTION
Renewable energy would meet an important proportion of the world needs in the future. Utilization of renewable energy can be broadly classified into two categories:
one is thermal application that converts renewable energy into thermal energy, and the other is electrical system that converts energy directly into electrical energy. Usually, these two utilizations are separately used with low energy efficiency. In order to solve this problem, a new combined system (electrical/Thermal) was developed which can generate both thermal and electrical energy simultaneously.
In 1978, Kern et al., [1] presented a prototype of PV/T system using air or water as a cooling fluid to decrease the temperature of solar cell. Florschuetz [2] extended Hottel- Whillier model to analyze the performance of solar energy hybrid PV/T system which could also provide domestic hot water. A system that employs a combination of different renewable energies has the advantage of energy balance and supply stability.
Hybrid geothermal solar system, can supply buildings with heat and electricity simultaneously because it uses renewable energy such as heat geothermal, solar thermal and photovoltaic. Hybrid geothermal solar system can also be constructed with a conventional energy system to improve efficiency and safety. In recent years, in order to reduce the cost of combined hybrid system, considerable researches were reported in the literature on new photovoltaic/thermal systems [3, 7].
In this study, geothermal and solar sources of heat are used as renewable energy sources. A small-scale setup of an hybrid geothermal solar system, consists of a geothermal heat pump for heating and cooling, solar collectors for hot water supply, reflectors, solar cells, absorber panel, back panel, heat exchanger, turbo compressor and incidental facilities.
* amaifi@umc.edu.dz
† bhioual_ouided@yahoo.fr
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2. SYSTEM MODEL
The system model is divided in two part: one that calculates the thermal heat system parameters such as temperature (T), thermal energy ( Q ) and thermal efficiency (th) of the sensor, and the second part is devoted to electric model, which calculates the electrical parameters of the solar concentrating photovoltaic/thermal air system as the current (I), voltage (V) the sensor performance and the generated electric power.
The solar thermal geothermic collector considered in this work is shown in figure 1
Fig. 1: The schematic model of solar thermal geothermic For the sake of simplicity, some Hypotheses are made as follows.
Heat transfer is one-dimensional and in a steady state in the direction of the flow.
The heat capacities of the glass, cylindro-parabolic concentrator, solar cell, fins, absorber and the insulating plate are negligible.
The parabolic concentrator is ideal and all the incident radiation in the acceptance angle can reach the solar cells.
The radiation passing through the interior of the parabolic concentrator is constant.
The solar radiation converted into thermal energy is completely absorbed by the panels and solar absorber.
The temperature of the solar cell and the absorber are uniform.
Based on these assumptions, the equations of energy can be written as:
For the glass cover
) T T ( A h ) A T T ( h
) T T ( h ) T T ( h ) 1
( C G
p g rpg c p ct g cpg
w g cgw s
g n rgs
2 g R g g
(1) Where n is the average number of reflection for radiation inside the acceptance angle, where Act, Acb and Ac are the areas (m2) of reflector, solar cell and convection, respectively, G is solar irradiance (W/m2) and C the concentrating ratio is 2.
At the Photovoltaic thermal plate
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) T T ( A h
) A T T ( A h A
) T T ( A h ) A T T ( A h
A
C 1 1
d P G )
P 1 C ( 1
d G
g p rcpg c g ct p cpg c ct
b p c rpb f cb
p p c cpf cb
pv n
2 g R n pv
p R g n
2 g R n p
g R g
(2)
The solar reflectivity g of the glass cover and pv of the photovoltaic cell surface and p of plate are assumed to be the same as that of the black absorber ( g= p =
pv ). Where d is a correction of gap loss. P is the solar cell packing factor with. The value of the glass transmittance g is 0.9, and plate absorptivity p, and glass absorptivity g are 0.9 and 0.06, respectively [8].
The heat exchanger
) T T ( A h
) A T T ( x h
d T d w
C m
1 f p cpfair c 1 cb f b air f cpf
fair
pair (3)
) T T ( A h
) A T T ( x h
d T d w
C m
1 f p cpfwater c
2 cb f b cpfwater fwater f
pwater (4)
Where mp is mass flux (kg/s), Cf is specific heat (J/kgK) and w is system width (m).
The insulating plate
) T T ( A h
) A T T ( h ) T T (
U rpb p b
c b cb f cpf a
b
b (5)
The back loss coefficient Ub is 0.0692 W/m2.K.
Electrical model
These three pairs of values were measured under standard test conditions illumination Gref 1000W/m2, temperature Tjref 25C [9]. Gref, is radiation reference (W/m2), Tjref, junction temperature reference (K).
1
T K n
) I R V ( exp q R I
I R I V
I
j s
s 0 i
sh
ph s (6)
Where Iph, Iccrph and I0 is the photocurrent, reference current of short-circuit and the saturation diode current (in A) respectively, Tj is the junction temperature of the cells (K) and qi is the charge of the electron, K is the Boltzmann constant, Eg is the gap energy (eV), is the ideality factor of the junction with values between 1-2, and
ref
I0 is a coefficient dependent on temperature and on the cell technology [10, 11].
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Performance parameters
Some main performance parameters of the solar thermal geothermic are defined and calculated as follows:
The useful thermal energy gained by the air flow through the system of the solar thermal geothermic is:
nj 1Qj nj 1mpCf(To,j Ti,j)
Q (7)
The thermal efficacity of the system is:
C G
Q t
(8)
The electric power produced by the system is [10].
V I
Epv (9)
The electrical efficiency of the system is:
C G Epv pv
(10)
The combined photovoltaic-thermal efficiency is the sum of PV and thermal efficiencies of the system [10]:
t pv tot
(11)
3. RESULTS AND DISCUSSION
Figure 2 shows the effects of the mass flow rate on the temperature of the fluid, the cells and insulating plate respectively in function of the panel length. It can be observed that temperature of the fluid and the cells, insulating plate, increase with the increase of the panel length. But Increasing of the air mass flow rate will decrease the temperature of the system, with constant sunlight radiance. This is explained by the fact when the quantity of air has to heat increase, involving a reduction in its temperature of exit.
Fig. 2: The mass flow influence on the fluid, cells and insulating plate temperature The figure 3 and figure 4 presented the effect of the mass flow on thermal and electrical efficiency of the system in the air flow direction. The efficiency (thermal and
25 electrical) increases with the air flow direction, the electric efficiency increase is linear and we have a small difference (higher) in the curves when the mass flow increasing in the range higher of 3 m on the air flow direction, and the thermal efficiency increases is exponential in the air flow direction, but this increase is very important when the mass flow increases (figure 4), because the internal convective exchangers have been improved and the heat losses remain constant when the air flow increases.
Fig. 3: Mass flow influence on the thermal efficiency
Fig. 4: The mass flow influence on the electrical efficiency
Fig. 5: the mass flow influence on the total efficiency 4. CONCLUSION
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In conclusion to this work, it appears that when the air mass flow rate increases, the amount of heated air rises. This causes a decrease in the outlet temperature which then cools, the solar cells system of the system. This indicates an improvement in the internal convective heat transfer. Consequently, the thermal and the electrical performance of the system grow. The increase of the electrical efficiency for solar cells following the decrease in there temperature, has a beneficial effect on the electrons and holes mobility. This carrier mobility increases as the temperature decreases. The results obtained suggest that it is a good alternative thermal electrical geothermal to conventional photovoltaic sensor installed separately. Heat extracted could use for heating the buildings or be transformed into another energy.
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