3.1. Building Description and Modeling
The considered specimen is a set of two small-scaled prefabricated buildings based at the International University of Rabat (see
Figure 2). It is mostly used to conduct research activities (materials of construction and insulation, integration of renewable energy sources, ICT for smart control and data collection, etc.) and is sometimes used as an office. A single prefabricated building accounts for 12 m
2 of surface and 30 m
3 of volume. It is mainly made of galvanized steel and an integrated foam polyurethane insulation in its lateral walls. As for the roof, it is made of galvanized steel, an air gap, and plaster. Finally, the flooring is made from galvanized steel and internal chipboard. The building is southeast oriented and contains one single-glazed window and a door on its south facade. More details about the characteristics of construction materials are presented in
Table 1.
Real registered data of Sala Al Jadida (latitude: 34.05°, longitude: −6.75°, elevation: 119 m) were taken into account for the simulated period.
Figure 3,
Figure 4 and
Figure 5 represent the external temperature, external solar radiation and internal solar distribution transmitted through the window, respectively. The shapes of the curves demonstrate that external temperature varies between a minimum of 6.48 °C and a maximum of 31.32 °C. This evolution is quite logical given the considered simulated period. As for external solar radiation, its varies following the alternations between day and night and might exceed 829 w/m
2 at the level of the south facade. Regarding internal solar distribution, the maximum value is in the order of 2.5 w/m
2 and is distributed almost equally on all facades, except from the west facade, which does not exceed 1.5. As for the ground temperature, it varies from 17.63 °C in December to 16.11 °C in January.
In this section, we aim to evaluate the thermal comfort inside the zone without any ventilation or air conditioning system activated. For this, the simulation was run using the EnergyPlus simulation tool. This tool is a commonly used building simulation program among researchers, engineers and designers. It is considered to be one of the most robust available simulation tools at both the commercial and academic levels. It combines all the entities of a building, starting with the envelope modeling, HVAC systems, passive systems and integrated renewable energy sources. The EnergyPlus program is based on the best capabilities of both DOE-2 and I-Blast simulation programs [
20]. Below are the assumptions that we considered during the simulations phase:
Version: 8.5;
Load convergence tolerance value: 0.04;
Temperature convergence tolerance value: 0.4;
Solar distribution: “Full interior and exterior”;
Maximum number of warmup days: 25;
Minimum number of warmup days: 6;
Surface Convective Algo: Inside: TARP
TARP, or the Thermal Analysis Research Program, blends correlations from AHRAE and flat plate experiments by sparrow et al. [
21].
Combination of MoWitt (measurement taken at the mobile window thermal test facility [
22] and BLAST detailed convection models:
Time step: 6 (calculation each 10 min);
Site ground temperatures: (values are being set according to experimental data);
December: 17.63; January: 16.11; February: 16.83;
Heat balance algorithm: Conduction Transfer Function (CTF);
Heat balance on the zone air is given as follows:
where
is the energy stored in zone air,
is the convective internal loads,
is the convective heat transfer from zone surfaces,
is the heat transfer due to interzone air mixing,
is the heat transfer due to the infiltration of outside air,
is the air system output,
is the internal heat transfer coefficient,
is the surface area,
is the surface temperature,
is the zone temperature,
is the mass flow,
is the infiltration mass flow,
is the system mass flow,
is the ambient infinite temperature, and
is the zone heat capacity, which can include thermal masses assumed to be in equilibrium with the zone air.
In order to solve the heat balance equation, EnergyPlus provides three main algorithms: the 3rdOrderBackwardDifference, the Euler Method, and the analytical solution. The first two methods use the finite difference method, in which the temperature difference is expressed as follows:
The Euler formula is mainly used to replace the derivative term of temperature:
In order to avoid instabilities, higher-order expressions with corresponding higher-order truncation errors were developed.
The major problem with this 3rdOrderBackwardDifference resolution method is that it has truncation errors and requires a fixed time step length for the first three time steps. For this, an analytical resolution was developed to obtain solutions without truncation errors and is independent of time step length. The temperature of the considered zone is then expressed as follows:
3.2. Earth-to-Air Heat Exchanger Modeling and Sizing
In this section, we refer to our previous work [
2] in which we have modeled and sized the Earth-to-air heat exchanger (EAHE) prototype. However, we present the main equations and the model characteristics and constraints.
In general, the dimensioning and sizing of an Earth-to-air heat exchanger primarily depends on five main parameters, namely: the burial depth of pipes, the air flow, the soil thermal properties, the exchanger geometry, and the pipes’ physical characteristics [
9].
Figure 6 depicts the heat balance of a buried pipe. It is characterized by its length and internal and external diameters. The soil thermal resistance was modeled as a cylindrical layer of soil around a tube with a thickness
p = 0.17 m [
10], also called the penetration depth or thermal boundary layer thickness.
Heat exchange between the soil and the air inside the pipe is reached through the combination between two major thermal phenomena: convection through the inside air and pipe wall and conduction through the pipe wall and surrounding soil.
In the following section, for simplification purposes, it is assumed that the contact between the pipe walls and the surrounding soil is perfect. Furthermore, the soil conductivity is supposed to be high enough compared to the surface resistance. In addition, the following assumptions are taken into account: the surrounding soil is a homogeneous medium with consistent physical properties and a constant temperature at 1.5 m [
10,
23], and the pipes are homogeneous.
The total heat transferred to the air when flowing through a buried pipe is given by [
23]:
where
is the mass flow rate of air (kg/s),
is the specific heat of air (J/kg-K), and
is the temperature difference (°C) between the inlet and outlet air temperatures through the EAHE.
The transferred heat, considering the thermal convection between the soil and the air, can also be given by:
where
is the overall heat transfer coefficient (W/m
2-K), and
is the internal surface area of the pipe (m
2). The logarithmic average temperature difference
is given by:
where
is the outlet temperature of the air,
is the inlet temperature of the air and
is the constant temperature of the soil.
in (10) can be expressed as follows:
where
is the convection resistance inside the pipe,
is the conduction resistance of the pipe’s wall, and
is the conduction resistance of the soil around the pipe. Thus, each resistance can respectively be presented by (13)–(15) [
23,
24].
where
is the convective heat transfer coefficient of the air (W/m
2 K),
L is the total length of the pipe,
p is the thickness of the thermal boundary layer (m),
and
are respectively the thermal conductivity of the pipe and the soil expressed in W/m. K. The convective heat transfer coefficient of the air inside the pipe is defined by:
where
is the thermal conductivity (W/m. K), and
is the Nusselt number. This latter number is calculated using appropriate correlations depending on the flow regime as detailed in [
25]. Thus, (16) depends on the Reynolds number and the shape and roughness of the pipe for turbulent flow. The Reynolds
number and Prandtl number
are expressed as follows:
where
is the dynamic viscosity of the air (kg/m. s) and
is the specific heat of the air (J/kg. K). The outlet air temperature is obtained by solving Equation (9) as a function of soil and inlet air temperature.
Calculating the pressure loss is required to size the ventilation system and to select the proper length and diameter of the pipe. Mainly, there are two types of pressure losses: the linear pressure loss
(Pa) and the singular pressure loss
(Pa). Linear pressure losses, or friction losses, comprise a complex function of the system geometry, the fluid properties and the flow rate in the system [
26]. They can be defined as follows:
where
is the friction losses coefficient (dimensionless),
is the volumetric flow rate of the air, and
is the density of the air (m
3/s).
As for singular pressure losses, they are also referred to as minor losses and are mainly due to entries and exits, fittings and valves. Furthermore, they represent additional energy dissipation in the flow, usually caused by secondary flows induced by curvature or recirculation. They can be defined as follows:
where
is the particular resistance losses of the pipe line (Pa).
The modeling and sizing of the system was calculated and simulated using MATLAB. Based on the above equations, we were able to compute the expected outlet air temperature using different pipe lengths and diameters. In what follows, we have conducted a parametric study for different pipe lengths and diameters to determine the best fit to our use, where the length and depth should not exceed 16 and 1.5 m, respectively.
It can be seen in
Figure 7 that the total pressure drops, also referred as the charge losses, are greater for long lengths and small diameters. This can be explained according to Bernoulli’s equations, which were modified to include head losses and pump work [
27].
where
is the pressure (Pa),
is the density (kg/m
3),
is the constant of gravity (m/s
2),
is the height above the reference (m),
loss is added by the pump (m), and
is the loss added by friction (m). The pressure loss due to friction translates the energy used in overcoming friction that is caused by the pipe walls. This parameter mainly depends on flow velocity, pipe diameter, length, roughness of the pipe and the Reynolds number.
Pressure loss presents the loss due to friction and is not related to the loss of total energy. This latter is conserved by the law of conservation. To see how diameter variation or length influences head loss, we analyze the equation of Darcy–Weisbach:
If the pipe length is doubled, the pressure losses will be doubled. Conversely, if a low rate and length are maintained as constant, the pressure loss is inversely proportional to diameter. In other words, if the pipe diameter is small, then the pressure drop will be high.
For diameters between 0.5 and 0.1 m, charge losses vary between a minimum of 0.01 Pa and a maximum of 500 Pa. These losses increase with length as well. To be more precise, for a diameter of 0.5 m, the charge losses increase from 0.01 to 0.07 Pa. As for a diameter of 0.1 m, the losses vary from 15 to 500 Pa. From this, it can be concluded that the diameter largely impacts the variation of the losses: the smaller it is, the greater the losses are.
Figure 8 shows the exchanged heat power through an EAHE system for different pipe lengths and diameters. The heat exchanged in the small pipe diameter is much greater than the larger pipes, and it converges to a certain constant value for longer lengths.
In
Figure 9, different airflows are presented, varying from 40 to 140 m
3/h with different pipe lengths. We decided to work on using two pipe diameters, varying from a minimum of 0.1 m and a maximum of 0.5 m. More airflow ensures more exchanged heat power. Moreover, a longer pipe gives greater heat exchange. However, it converges to a constant value when it exceeds 100 m for a pipe diameter of 0.5 m and a ground temperature of 20 °C. The same behavior is observed for a pipe diameter of 0.1 m, but this time with a shorter length of approximately 60 m. From this, we can conclude that by combining a small diameter and high airflow, we can gain more heat exchange with medium-length pipes.
Figure 10 presents the total thermal resistance response to different pipe diameters and lengths. The global behavior shows that the thermal resistance decreases when the pipe’s length increases. As for the diameter, the smaller it is, the weaker the resistance. In other words, heat exchange is greater when choosing a small diameter. This is especially true when the pipe’s length does not reach 30 m. Because once it does, the thermal resistance converges toward a small value, if not null.
Figure 11 shows the power needed to satisfy the heat transfer and the airflow required at the outlet. It is clear that for tighter pipes, more power is needed, and more is required for longer pipes. However, it converges to a nearly constant value when reaching more than 60 m of length and 20 °C of ground temperature.
The results of the system sizing and modeling simulations were calculated by taking into account certain parameters such as the space limitation of the test site as well as the specifications from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) regarding air quality inside buildings. The simulations were run for an average ground temperature of 20 °C and a pipe length of 17 m, a building volume of 26 m
3 with a minimum of three air changes per hour. This air change rate is in line with the recommendations of (ASHRAE), which states a minimum of 7.5 L/s per person in a building [
28]. Based on the ASHRAE Equations (23) and (24):
where
is the volumetric flow rate of air in L/s for a given number of air changes per hour
and a certain space volume
.
where
is the ventilation rate per person in L/s,
is the air changes per hour, D is the occupant density in m
2/occupant, and
is the ceiling height in meters.
The volumetric air flow
that was used as a constraint in our model was calculated using the above-mentioned formulas with an
air changes per hour,
(two occupants in 12 m
2) and
= 2.20 m for ceiling height. The resulting ventilation rate per person was 11 L/s, which is greater than the minimum ventilation rate per person (7.5 L/s) required by ASHRAE. Using Equation (23), the volumetric air flow rate was about 80 m
3/h, which is the minimum that complies with the ASHRAE requirements. From
Figure 12, more air flow requires more ventilation power and subsequently produces more charge losses while affecting the desired outlet temperature of the system.
Given the concluded volumetric air flow rate, the resulting heat power that was exchanged for a 17 m length and 0.1 m diameter pipe was 530 W (
Figure 8). The ventilation power required for such length, diameter and volumetric air flow was about 4 W (
Figure 11), considering a charge loss of approximately 175 Pa (
Figure 7).
Figure 13 shows the EAHE model outlet air temperature for both use cases of summer and winter. We have considered, in this phase of simulation, a severe condition for the zone area of Sala El Jadida city in Morocco where the temperature of the inlet air hypothetically reaches 45 °C in summer and 5 °C in winter. As can be seen, during the summer period, the system can bring down the inlet temperature from 45 to nearly 25 °C with a difference of 20 °C from inlet to outlet for a pipe diameter of 0.1 m.
The difference in temperature between the pipe diameters varied between 1.5 and 2 °C for a 17 m pipe length. For the winter period, where the EAHE works as a heater, it is shown that the system helps to increase the inlet temperature from 5 to 20 °C with a difference of 15 °C for a pipe diameter of 0.1 m. The temperatures outlets for the different pipe diameters remain in the same interval as the summer period.
The results of this study highlight the required parameters, such as the appropriate diameter of PVC pipe as well as the required length, air flow and ventilation power. We have also placed more emphasize on the materials constituting the system pipes, namely, their microstructures and their properties. After several studies and calculations taking into consideration the actual context of our test site, we have deduced that the thermal resistance of the pipe’s material does not significantly influence the desired outlet air temperature as shown in
Figure 12 (right).
Meanwhile, an analysis of the material and installation costs was realized to maximize quality while maintaining a minimal budget. We decided to use PVC pipes for their low cost and thermal characteristics. PVC pipes are easy to maintain in case of damage and due to their affordable cost. Moreover, they have an acceptable thermal conductivity, also called the “K value”, of 0.26 W/mK. Simply, this type of pipe is capable of transferring heat at a rate of 0.26 watt for every degree of temperature difference between opposite faces. One face is in contact with the underground soil and the other with the air inside the pipe. PVC pipes are insensitive to atmospheric pollution, salty air and climate aggression.
The Earth-to-air hear exchange prototype was deployed based on the above-mentioned constraint but also by depending on the test site where it was implemented. Several limitations affected our final prototype sizing, mainly the available space and depth.
Figure 14 shows the surface measurements as well as the sensors placement. The surface is 5 m long, 1 m wide and approximately 1.2 m deep. The pipe is about 17 m long and is a serpentine structure with a 0.5 m curved intake. The surface of the test site is covered with a layer of grass and is frequently irrigated.
3.3. Economic Modeling
The main reason behind the increasing energy consumption in buildings, whether residential or tertiary, is the excessive usage of heating, ventilation and air conditioning (HVAC) devices. This is mainly due to the continuous search for thermal comfort, hence an increase in energy bills. The levelized cost of energy/electricity (LCOE) [
29,
30], is a measure of the average net present cost of electricity generation for an engendering plant over its lifetime. LCOE is calculated as follows [
30]:
where
is the investment expenditures in the year t,
translates operations and maintenance expenditures in the year t,
is the fuel expenditures in the year t,
is the electrical energy generated in the year t,
is the discount rate, and
is the expected lifetime of the system or power station. Inputs to LCOE are selected by the estimator. They can consider the cost of capital, discharging, fuel, fixed and variable operations, maintenance, and financing as well as an assumed utilization rate.
For our case, the investment expenditures in one year are about MAD 20,000 (USD 2000). As for the operations and maintenance cost (fan, filter, sensors, irrigation), the cost was about MAD 1000 (USD 100). Concerning fuel expenditure, the amount of expenditure required to maintain adequate temperatures in the home and to meet other energy needs was taken as the price of the power needed for the fan to work over the year.
If we consider a maximum functioning case of the fan, we can assume that the fan uses a power of 4 to 6 watts. It also uses power for monitoring and data logging (Nvidia with Raspberry pi and microcontrollers with access points and switches), which totals a power of 50 watts. Hence, for an overall consumption of 438 kWh per continuous scenario per year with a price of 1.62 MAD/kWh (0.16 USD), the result is about 710 MAD/year (71 USD).
The power gained by using the EAHE system is about 1.4 kWh daily, resulting in 511 kWh per year and a savings of 827 MAD/year (82 USD).