The Seasonal Storage of Thermal Solar Energy in Iraq - IJAIEM

International Journal of Application or Innovation in Engineering & Management (IJAIEM) Web Site: www.ijaiem.org Email: [email protected] Volume 3, Issue 3, March 2014

ISSN 2319 - 4847

The Seasonal Storage of Thermal Solar Energy in Iraq Al-Sudany Naseer K1, Al-Sa'ady Ahmed F2, Al-Bahadly Fadhil M.3, Al-Sudany Alaa H.2 1

Ministry of electricity, Renewable Energy and Environment Center. Al-Mustansiriyah University, College of Education, Physics Department. 3 student, Al-Mustansiriyah University, College of Education, Physics Department. 2

Abstract This research deals with storage of the thermal solar energy in the soil in summer season (from May to Oct) for latter utilization (in winter) to reduce the energy consumption. The quasi-three-dimensional model and finite line source model are used to compute the heat transfer from the used solar water heater at 90 oc of active area 4.16m 2 to the underground using U-tube with length of 12m and diameter 0.1m, for storage the thermal solar energy of the summer season to use it in winter for supplying heat water and space heating of domestic consumption. The results indicated that the capacity of stored thermal energy in the soil is 38% of the total collected energy, which equal to 1.013Mwh in the soil of Al-kreaat quarter in Baghdad. The energy saving per the borehole length is 84.8 kWh/m, the result also show that the amont of storage energy depended on the borehole length.

Keywords: seasonal heat storage; quasi-three-dimensional model; borehole; solar collector

1. Introduction The using of fossil fuel causes many effects on the environment such as the greenhouse effect which causes increase in the temperature of the earth because emission the CO2 and these sources are limited and predicted to become scarcer and more expensive in coming years [1]. The renewable sources would help to reduce the emission of greenhouse gasses. One of an important source of renewable energies is the sun. The thermal energy storage (TES) can be used to reduce the fossil fuel consumption by storing of huge solar thermal energy in summer for later utilization in winter season as shown in figure (1). Borehole thermal energy storage (BTES) is a method for storing thermal energy in the underground. The borehole used as ground heat exchanger (GHE) for the thermal energy which collected by the solar collector and transfer to the ground by circulation the fluid in the U-tube. Thus, the present work aims to store thermal solar energy in soil and compute the quantity of thermal energy that can be stored around the heat exchanger (grout) along six months of the summer season in Algreaat quarter in Baghdad.

Figure 1: System connection (a) in summer, (b) in winter

2. Solar collector The solar radiation that can be absorbed by the evacuated tube collector and converted to thermal energy on an hourly basis is depending on incident solar radiation , area of the absorber and the collector efficiency . The amount of energy stored in the fluid of collector's tank can describes as [2]: . The temperature of the fluid inside the tank can be evaluated by the balance equation. ( - ) Where m is mass and specific heat of heat transfer fluid HTF, is initial fluid temperature and temperature. The final fluid temperature in the solar collector tank is

. is final fluid

The average daily efficiency of an evacuated tube collector was found approaching 60% [3]. In summer season, the appropriate tilt angle for the collector system in Baghdad is 15o [4].the solar evacuated tube collector specification is shown in table1.

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Table (1): The solar evacuated tube collector design Tube Length (m) 1.8 Tube Radius (m) 0.058 No of tubes 40 Distance between tubes (m) 0.042 Collector Area (m2) 4.16 Collector efficiency 60% Collector tank Length (m) 4 Collector tank Radius (m) 0.098 Collector tank Volume (liter) 120

3. Solar Radiation Incident solar insolation on the collector of 4.16m2 at tilt angle of (15o) in Baghdad for the months ( May, June, July, Aug, Sep and Oct ) are shown in table 2 [5]. Table 2: Solar Insolation in Baghdad at tilt angle of 15o [5] month kWh/m2 Per kWh/m2 Per day month kWh per month May 6.38 191.4 477.73 June 6.46 193.8 483.72 July 6.46 192.3 479.98 Aug 6.16 184.8 461.63 Sep 5.54 166.2 414.83 Oct 4.58 137.4 342.95 Sum 35.53 1065.9 2660.84 For the whole system, the resulted thermal energy from used solar collector for six summer months is 2.66Mwh. Daily the gained thermal energy is used to raise the temperature of 120 liter of HTF to 90oc. This fluid will be circulated by the pump of power 0.37 kW with flow rate of 0.08 kg/s through borehole tube of 12 liter capacity. According to equations 1 and 3 the HTF (120 L) in May, June, July and Aug will be circulated for 4 times every day, this corresponds to 40 cycles per day through borehole. In Sep and October the HTF Aug will be circulated for %times and 8 times every day respectively. However, the on-off heat pump will be controlled by temperature sensors (thermocouple) which put in HTF tank.

4. Heat transfer inside borehole The borehole is consisted of two pipes made of high-density Polyethylene with a center-to- center distance 2D, often called the shank spacing, the space between the pipes and the borehole wall are usually filled with a grout (soil) as shown in figure 2a. The grout is used to augment heat transfer from the fluid to the ground [6].In the GHE, the HTF flows along the borehole in one channel down to the bottom of the borehole and back upward in another channel, while exchanging heat with the ground. A two dimensional horizontal cross section of a U-tube borehole is presented schematically in Figure 2b.

Figure 2: Schematic diagram of a-grout borehole, b-Thermal Resistance in the borehole [7] The actual heat transfer processes occurring in a single U-tube ground heat exchanger include: (1) the convective heat transfer between the HTF in the U-tube and the tube wall; (2) the conduction of heat in the tube wall; and (3) the conduction of heat in the grout and the ground soil [8]. The heat transfer from the circulating fluid in the pipes to the surrounding ground can be describe by a quasi-three-dimensional model was proposed by Zeng et al [9].The main objective of the model is to determine the entering and leaving temperatures of the circulating fluid in the exchanger according to the borehole wall temperature and its heat flow [9], and taking into account the fluid axial convective heat transfer and thermal “short-circuiting” among U-tube legs[8]. Being minor in order, the conductive heat flow in the grout


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and ground in the (z) direction is neglected to keep the model concise and analytically manageable. The energy balance equations for up-flow and down-flow of the circulating fluid can be written as [7].

Where Tf1 , Tf2, Tb , and H are the temperatures of the fluid running downwards, the temperatures of the fluid running upward, the temperatures of the borehole wall, mass flow rate, heat specific and borehole length respectively . are dimensionless thermal resistance. .

Here, R11 and R22 are the thermal resistance between the circulating fluid and the borehole wall, and R12 is the resistance between the circulating fluid in the pipes [10]. In most engineering applications, the configuration of the Utube in the borehole may be assumed symmetric, and here it is assumed that R22=R11, therefore,

Hellström (1991) presented a technique to evaluate R11and R12 based on the line source solution for each pipe [11]:

Where k and kb are the ground and grout thermal conductivities, respectively ,rb is the borehole radius, rp is the outer radius of the pipe, Rpipe is the heat transfer resistance from the fluid inside the U-tubes to the pipe outer surface (considered constant along the borehole depth), which combines the fluid convective resistance (Rfluid), the pipe resistance in the pipe (Rp) and a contact resistance associated with gaps between the pipes and the grout (Rair) can be calculated with Equations [6]:

where

is the inner pipe radius, the fluid resistance can be calculated using Equations [12]:

Where is the Heat-transfer coefficient, rip is the inner pipe radius, kp is the pipe thermal conductivity, hi is convective heat transfer coefficient inside the U-tubes, and Rair is a contact resistance at the grout/pipe interface. The resistance was set to zero in this work, the value of hi is assumed to be the same in both circuits and constant along the depth of the borehole. Two boundary conditions are necessary to complete the solution

Where is the temperature of the fluid entering the U-tube, and together are equal at end of U-tube. Zeng et al (2003a) formulate the temperature profiles of the fluids flowing in the downward pipe in the borehole [10]:


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The temperature profiles of the fluids flowing in the upward pipe in the borehole

Where the dimensionless parameters are defined as

Where is a fluid temperature for any point per length, the parameter P is always larger than zero, but it is always smaller than 1, that is ( 0