Thermal Efficiency for Passive Solar Chimney with and Without Heat Storage material

In this study, a different design of passive air Solar Chimney(SC)was tested by installing it in the south wall of insulated test room in Baghdad city. The SC was designed from vertical and inclined parts connected serially together, the vertical SC (first part) has a single pass and Thermal Energy Storage Box Collector (TESB (refined paraffin wax as Phase Change Material(PCM)-Copper Foam Matrix(CFM))), while the inclined SC was designed in single pass, double passes and double pass with TESB (semi refined paraffin wax with copper foam matrix) with selective working angle ((30, 45 and 60). A computational model was employed and solved by Finite Volume Method (FVM) to simulate the air induced through the test room by SC effect. The governing equation of Computational Fluid Dynamic (CFD) model was developed by the effective heat capacity method equation to describe the heat storage and release from PCM-CFM. Practical and computational Results referred to increase in thermal conductivity of the paraffin wax that supported by CFM than 10 times, while the ventilation effect is still active for hours after sun set amount. The maximum ventilation mass flow rate with TESB collector was 36.651 kg/hr., when the overall discharge coefficient equals 0.371. Also, the experimental results referred to the best working angle range 45~60, while the highest approaching temperature (between air and collector) was appeared for the double passes flat plate collector. Results gave higher heat storage efficiency 47% when the maximum solar radiation 780 W/m at 12.00 pm, and the energy summation through duration of charging time was 18460 kJ. Double passes SC at 60 angle presented the highest efficiency with value approaching to 73%, while TESB collector efficiency depicted highest efficiency value 70% at 12:00 pm.

There are many studies carried out to decrease percentage of the carbon release by consumption from traditional energy, and those have been conducted by employing the renewable energy. Solar irradiance is one of the most important energy sources, especially in the day time. But, the decrease in the solar irradiance near the sun-set, or absence of the energy after sun-set, leads to searching for method to store the thermal energy through day time and release it after sun-set or in the night time. Most of studies (experimental or numerical or both) employing solar chimney for heating or cooling and ventilation without using heat storage, calculated the instantaneous efficiency only; for different effects, many of these studies estimated the effect of heat storage with time, or estimated the instantaneous efficiency for solar chimney and neglected the heat storage, and at the same time they avoided estimating the real thermal efficiency in the periods of charging and discharging heat in the thermal energy storage material. So in this study, the thermal efficiency of solar collector is analyzing through the periodic time for charging and discharging depending on experimental with computational results. 3 2. EXPERIMENTAL SET-UP 2.1. Rig set-up Three different types of solar chimney were tested with ventilation mode. SCs types are single pass and double passes flat plate collector, and double pass TESB collector. Those different SCs were installed to the south wall of insulated test room. To enhance heat transfer inside the thermal storage material for tilted SC, so semi refined paraffin wax with melting temperature range (48~54℃) was supported by copper foam matrix with 95% porosity and porous per inch (ppi) equal to 30. A vertical preheat SC with full refined paraffin wax and CFM were installed before the tilted SC with( = 58~62℃). The vertical SC was supported by array of evacuated tubular collector with a thermosyphon to collect the solar irradiance and charge TES material, also a direct irradiance was collected by the face surface of vertical TESBC, as shown in Figs.1, 2, and 3.

Measurement and instruments tools
Data acquisition system with more than 100 thermocouple probes was used to read the temperature in each surface of collector, inside thermal storage material, temperature of air through the SC, transparent acrylic cover and else. Also, a high accuracy digital anemometer was employed to measure the air velocity in different locations and different levels through solar chimney gap. Modern solar intensity readers with weather station were installed to the test room.

Effective dimensions of the system
The most important effective dimension was the aspect ratio (ar) of SC (ratio of solar chimney length to its gap). The ar design value was depending on the optimum value at 12/1. So, the final design value for ar equals 25/1 for single pass SC and 50/1 for each pass of double passes SC. Depending on the designed value of ar, Fig. 2, so other dimensions of SC will be limited. For tilted SC, the length was 1.5 m and width 0.75 m, chimney gap equals 6 cm in single pass and 3 cm for each pass in the double passes SC. The Thermal Energy Storage Box (TESB) (PCM-CFM) in vertical SC has dimensions 1*0.75*0.025 m, while the tilted TESB has dimensions 1.5*0.75*0.025 m, and the weight factor (wax / combined) represents 59.48%.
The insulated test room has dimensions (2*1.5*1.5) m, supported by SC's in south wall. Length of the test room is bigger than width and height, to clarify the behavior of temperature distribution inside room. Structure of test room consists of a wooden frame, insulated by cork and glass wool.

THE SOLAR CHIMNEY (SC)
The system of ventilation that used SC consists of a vertical SC and tilted SC, both of them are connected together in series. Vertical solar chimney (preheat SC) depended on the evacuated tubular collector with a thermosyphon beside the transparence covered collector that was employed to collect the heat from the solar irradiance, Fig. 3. In vertical collector, the heat storage inside the combined of PCM-CFM. In tilted solar chimney, three different types of collector were used as mentioned previously. Single pass flat plate collector is show in Fig. 5   plate collector in Fig. 6, and double passes flat thermal energy storage box collector in Fig. 7. The third collector depended on the PCM-CFM to store the heat and release it after sun-set.

3.1Thermal energy storage material
To store the heat energy and release it in active time, the heat must be transferred in a limited duration time, and that means limit the value of thermal conductivity of the storage material. So, a CFM with 95% porosity and 30 porous per inch (ppi) was employed to support the main storage material (paraffin wax), Fig. 4.

Effective thermal conductivity
The effective thermal conductivity for the combined PCM-CFM is defined by the Eq. (2) The effective thermal conductivity is a result of these four layers being placed in parallel, this thermal conductivity is calculated for each section (n: A, B, C, D) or (n:1,2,3,4) for the tetrakaidecahedron (the structure of a tetrakaidecahedron, which is a fourteen-face polyhedron comprising six squares and eight hexagons) Fig. 8. By using such a polyhedron approximation, (Boomsma & Poulikakos, 2001) obtained a good agreement between model predictions and experimental data on metal foams with porosities from 88% to 98%. The thermal conductivity through the representative section is calculated based on heat conduction through a series of four levels using Fourier's law of heat conduction to give the relation.
Substituting the equation for the section lengths (L n ), the thermal conductivities for each (k n ), and the positive solution for d from Eq. (3) (4) Introducing the simplifying notation gives, Finally, this yields the final result of the effective thermal conductivity to be: Where, e = 0.339, k s : Thermal conductivity of the material used to manufacture the metal foam. k f : Thermal conductivity of the material saturated in the metal foam. R A , R B , R C and R D : Thermal resistance of four different layers inside a tetrakaidecahedron cell. The effective thermal conductivity k ̅ PCM−MF is a result of these four thermal layers placed in parallel.
Other properties of the combined, like density and specific heat, can be estimated from the general equations as shown below (Vadwala, 2011): = (1 − ) + (10)

Equivalent heat capacity
To calculate the effective specific heat for paraffin wax in the mushy zone Fig. 9, as shown below; So, by defining the Equivalent Heat Capacity Method, If the latent heat of fusion is known, and specific heat for solid and liquid with ∆ , the specific heat of mushy zone Eq. (13) can be defined as shown in Fig. 9. To estimate the heat, transfer by conduction or heat storage in materials, so it must be defined the properties of materials that consist of the galvanized steel, acrylic, copper foam matrix and the properties of paraffin wax at solid and liquid state.
To simplify the computational model, the physical fact that the phase change taken place in a temperature interval ( ± ∆ ) can be employed. So, the heat balance equations that describe the heat storage and heat transfer during phase change can be divided as regions state: solid, liquid and mushy zone.
Many methods are defined to describe the phase change. The effective heat capacity method is the one of the simplest and effective methods to estimate the heat transfer or heat storage in the different three regions. With a narrow temperature difference in mushy zone, this method depicts the most precise method.

Effective heat capacity method
In this method, the latent heat effect is expressed as a finite temperature dependent on the specific heat that occurs over the temperature range. In this method, it is possible to describe the nonisothermal phase change. So, the effective (equivalent) heat capacity during the phase change can be presented as shown in the equation (Ahmed, 2010): For incompressible fluid and for solid, it is possible to change instead of as shown above and with acceptable error.
To define the new set of properties of PCM by melting process for a small unit volume, Fig. 10 shows the time-dependent temperature of unit volume of phase change material during 1 and 2 . The density of PCM can be estimated by arithmetic average: Eq. (30) can be re-written as:

Simultaneous efficiency of heat storage
The simultaneous efficiency at the duration time of heat charge, can be compute from the rate of heat storage in TESB during its exposure to the solar radiation which can be calculated from the eqn. below; = ∆ℎ/ =( ∆ℎ + ∆ )/ Where, m= mass for wax and copper foam matrix. t: duration time to charge TESB by heat (q).
So, the heat storage in the duration time t can be found from Eq. (34) = .
The heat storage during the duration charge for N time can be computed by algebraic summation of Eq. (33) to get Thus, the ratio of the heat storage to the heat absorbed by TESB is

TILTED SOLAR CHIMNEY EFFICIENCY
The tilted SC efficiency in ventilation mode with different collector configuration and in different inclination angles, is evaluated by employing the data result as shown below: With assumption of no heat losses (small value) with good heat insulation.

The efficiency for double passes SC collector
Same as the calculation in the previous collector.

The efficiency for double passes TESB SC collector
There are three different evaluation values depending on change of solar irradiance with time and heat storage or release from the combined thermal storage material. The concluded diagram from this study in Fig. 11 depicts the three different evaluation zones.
Heat storage can be evaluated from the graph of energy storage efficiency.

EXPERIMENTAL TEST PROCEDURE
The experimental work was achieved in June, 2015, where data are logging for the dependent variables along 12 hours. All measurements and instruments tools were used as mentioned previously to measure and record the dependent variables, for each 2 hours as test interval, while the average value for general variables were measured and recorded by weather station data logger along 24 hours, with 20 minutes as test interval. Temperature was Measuring in chimney gap, test room space, TESB and outdoor respectively.

THEORETICAL CONSIDERATION
In this study, it is assumed that there are no heat losses directly from test room or from the insulated part of the solar chimney to the surround. So, according to the heat balance, the heat absorbed by collector must be equal to heat released to the working air that passes through the chimney gap for the flat plate collector in single and double passes. While in the TESB collector, the heat balance means the summation of heat released to the working fluid plus heat that stored or rejected from the compound thermal storage material. Fig. 12 reveals the heat storage efficiency with maximum value at 12:00 pm, because the higher difference in temperature between inside TESB and its up-surface collector ( Figs. 16, 17 and 18, a comparison for collector's temperature between different SC types and at the same working angle is presented. The working angle at 30 o showed that the increasing in collector temperature for single phase collector is more than other types (with low approaching for air to collector temperature), and the double passes flat plate collector is the lowest in temperature (with highest value in approaching). That means the single pass had a bad attachment between air and collector surface, while the double passes flat plate collector had a good attachment between air and surface collector (good heat transfer). Also, the highest value of aspect ratio (ar) gives a guarantee to prevent back air flow (movement) in chimney gap and also induced air flow over surface collector with good contact (good heat transfer). TESB collector, storage heat energy that releases after sunset for long time. So, it still has higher temperature after sunset and provides the system by the heat needed. Figs. 19, 20 and 21 present the thermal efficiency for the three types tilted solar collector with different angles. The results appeared clearly the positive effect for the angle, that means of at vertical SC, the difference pressure increases and leads to increasing in the air flow. Double passes SC at 60 o angle presents the highest efficiency with value approaching to 73%. TESB collector in Figure 21 depicts the highest efficiency value 70% at 12:00 pm. One of more important notes, the double passes SC with TESB collector effect remains after sunset for a long time as shown in Figs. 22, 23 and 24, while the highest efficiency of storage energy was recorded at 60 o angle. Also, the increase of efficiency after sunset occurs, because the sharp decrease of outside temperature comparing with the hot collector leads to increasing the solar chimney efficiency.

CONCLUSION
Heat storage in the phase change material like wax is not efficient, because there is a bad thermal conductivity and that means bad heat transfer. To enhance the thermal conductivity, it is effective to employ CFM with high porosity to enhance the heat transfer. Heat storage in the combined material still releases heat energy after absence the heat source, and that refers to success for employing the paraffin wax with CFM as a TESM. To estimate the SC efficiency, a heat balanced equation between the solar irradiance and the heat that charge and release from the combined material PCM-CFM must be made during the experiment time (before sun rise, during day time and arriving to the night time (after sun set)).