1. Introduction
The actual global shift in energy usage nowadays is towards a decrease in fuel consumption, thereby reducing harmful carbon dioxide emissions. This can be achieved by energy management and renewable energy [
1,
2,
3,
4,
5,
6]. In particular, heat recovery represents a key aspect in the notable growth of energy management efforts [
7,
8,
9,
10,
11]. The vast range of applications of heat recovery includes ICEs—internal combustion engines [
11], chimneys [
12,
13,
14,
15,
16], energy recovery from shower water drain [
17,
18], and HVAC (Heating Ventilating and Air Conditioning) [
19,
20]. Heat recovery is based on the idea that waste heat generated by a process is used for either heating purposes or non-heating purposes. Waste heat appears in several applications, especially in manufacturing processes [
21,
22].
Moreover, heating, ventilating, and air conditioning systems (HVAC) are no longer a luxury but a necessity for citizens and thus require a variety of energy systems that need to be handled increasingly [
23]. However, HVAC systems account for one-third of energy consumption for society. As a result, the aim to reduce the energy load of the HVAC system and consequently reduce the environmental impacts and costs has become a topic of interest for many researchers. Studies have mainly focused on controlling the flow of air using VAV (Variable Air Volume) or VRV (Variable Refrigerant Volume) methods in response to the variations of energy loads in demand, while other studies have focused on improving the efficiency of specific equipment such as coils or refrigerators in order to increase the efficiency of the overall system.
For example, Mancini et al. [
24] evaluated the building energy requirements and the Indoor Environmental Quality while varying the mass flow rate of the HVAC system, which is provided by air-handling units. The results showed that a 50% reduction in the air flow rate will have a significant reduction of energy consumption of the building by up to 45%, with a negligible impact on the IEQ. As a matter of fact, one of the important solutions to reduce energy requirements is to have efficient control of the HVAC system flow rates, especially during low-occupancy periods.
Nevertheless, cooling outside fresh air represents 20–40% of the total energy consumption for HVAC in hot weather conditions, for example, and the percentage can even attain higher values if the demand for fresh air increases in critical situations, such as in hospitals, kitchens, or factories. Heat recovery from air exhaust to fresh air is considered a crucial step towards reducing the energy needs of a building. The common methods are mainly divided into two categories: energy wheels and static heat exchangers.
Examining the literature, it is noticed that studies that deal with heat recuperation in all-air HVAC setups are scarce. Moreover, studies on the recovery of heat from HVAC systems’ exhausted airflow (cold/hot) are mainly on a large scale, using thermal wheel or a fixed plate heat exchanger. In this context, the present study concerns cooling/heating fresh inlet airflow using the exhaust cold/hot airflow from HVAC setups. The idea, for example, for a conditioned room in a cold climate is to heat or preheat the fresh outside air by utilizing the drained hot air from the room.
All-water, all-air, and water–air are the three basic configurations of HVAC technologies. All-water-type distributes heat or cool water for the conditioned space using pipes, while, in water–air systems, water and air are both utilized to heat or cool a certain space, whereas the all-air type has gained this terminology in HVAC because the regulation of humidity and temperature is done by air supply only.
Figure 1 illustrates the working principle for this HVAC system type [
23].
The configuration of different parts is presented in
Figure 1a. The “Air Handling Unit” (AHU) is intended to add or take away energy from airstreams prior to their supply to air-conditioned spaces [
23]. In fact, it enables air to be heated, cooled, humidified, dehumidified, cleaned, and circulated to different air-conditioned areas in the zone. Additionally, outside air is introduced and inside air is extracted by means of the AHU. Thus, to balance between efficiency and comfort, a portion of the return air outside of the air-conditioned area is drained, and then, an equivalent quantity of outdoor air is combined. This permits a constant supply of fresh air to the space and mitigates the energy requirements for the let-in air to be totally conditioned from the outside air.
Ramadan et al. [
25] performed a parametric study on HVAC systems with air–water heat recovery. A counter-flow concentric tube heat exchanger at the condenser is used. Heated air downstream the condenser warms up the tube water. The results showed that the outlet temperature of the water rises from 31 °C to 74 °C for an increase of the cooling load from 3.52 to 63.31 kW. Yau et al. [
25] performed a heat recovery study using a heat pipe heat exchanger (HPHE) from an all-air HVAC setup. HPHE with two, four, six, and eight rows were tested. The coil face velocity was 2 m/s, while the temperature of the return air was 24 °C. HPHE results were recorded over one week. It showed an overall yearly savings of 2885 kWh, which can be raised to 7023 kWh per year if an eight-row HPHE is used. In addition, the volumetric flow rate will be increased, and a higher amount of energy recovery is expected as the face velocity is increased, but this results in a higher-pressure drop in the coil, which decreases the effectiveness. Shen et al. [
26] investigated a fixed-plate heat recovery setup for a liquid desiccant regenerator. Process prediction, testing, and discussion with heat recovery was facilitated by a simple heat and mass transfer model for the fixed plate. The predicted results and the investigated values were compared. The experimental results confirmed the predicted values with an error rate lower than 11% for all performances. Additionally, 16–19% of the total consumption of energy was recovered from the outlet air, which led to about 14–18% savings of energy. Nasif et al. [
27] studied how an air conditioning system is affected when equipped with an air-to-air fixed plate heat exchanger for energy recovery. Multiple counter-flow configurations were investigated, namely, L-shaped, Z-shaped, and Z-shaped opposite flows. The experimental results showed that the L-shaped heat exchanger provided energy savings up to 20,833 kWh in comparison to the two other layouts. Moreover, the recovery of the L-shaped exchanger was up to 25,000 kWh more than the other configurations.
This paper suggests a heat recovery concept that is based on preheating/precooling the cold/hot fresh outside air by means of the relatively hot/cold exhaust air in cold weather conditions. It is attained at this time by a compact air-to-air crossflow heat exchanger (applicable on the scale of large and small buildings also); its two airflow streams are the outside and exhaust airflows, as shown in
Figure 1b.
The remainder of this manuscript is ordered as the following:
Section 2 provides the materials and methods,
Section 3 exposes the results and analysis,
Section 4 presents an economic and environmental study, and lastly,
Section 5 exhibits the main conclusions.
2. Materials and Methods
For the investigation purposes discussed in the preceding section, an experimental implementation of the suggested heat recovery setup was made. The prototype consists mainly of a wood box that represents the conditioned space (room). This box has a volume of 6.7
, and its walls have a thickness of 15 mm and a thermal conductivity of 0.15 W/mK. Furthermore, an air-conditioning split system of 5.3 kW-capacity is utilized to supply the cold and hot air to the chamber, and the heat recovery system is ensured by the ducting system. This ducting system is composed of a duct that is isolated (adiabatic), fin-and-tube heat exchanger of a crossflow type (air-to-air), and a variable speed fan.
Figure 2 shows the schematics of this implemented prototype.
The air speed (and then flow rate) and temperature are measured at different locations for the purpose of testing the system’s performance. The five temperatures in the duct downstream and upstream of the fin-and-tube heat exchanger, at the heat exchanger’s inlet and outlet, and in the box are read continuously by using digital type K thermocouples. The air velocities at the two outlets of the heat exchanger are measured by using two anemometers. One of the two speeds is used to calculate the air flow rate through the heat exchanger (through the tubes); the second one is used to calculate the air flow rate passing the geometry of the heat exchanger (through the fins).
On the other hand, the outside cold and hot climates (outside airflow) are simulated using a compressor and a turbine fan, respectively.
It should be noticed that the conditioned space is equipped with two openings (
Figure 2): one at the bottom of the room for outdoor hot weather simulation conditions and another one for outdoor cold weather simulation conditions. These openings are used in each case to connect the heat recovery system to the conditioned space at the appropriate locations of the presence of hot and cold air for the return.
The air mass flow rates flowing in the exchanger
and through it
are estimated as:
where
is the density of air,
and
are, respectively, the heat exchanger outlet cross-section area and the duct’s cross-section area, where the heat exchanger is placed.
and
are the average velocities of air at the heat exchanger outlet and on a plane of the duct downstream the heat exchanger, respectively.
The heat gained or lost by the airflow in the heat exchanger and through it is estimated by using the equations below, respectively:
where
is the heat capacity of air, and
and
are the exchanger’s inlet and outlet air temperatures, respectively.
is the air temperature downstream the exchanger, and
is the air temperature upstream the exchanger.
Finally, the effectiveness “
” of the fin-and-tube exchanger is represented as a fraction of the real heat exchanged over the maximum possible heat exchange and estimated by the following equation:
where
,
,
, and
are, respectively, the mass flow rate, heat capacity, and hot fluid’s outlet and inlet temperatures.
is the cold fluid’s inlet temperature.
Four tests are done, and the operating conditions, along with air properties, evaluated at the average temperature between the indoor and outdoor conditions are represented in
Table 1. Two tests were performed in which the exchanger flow rate was variated and duct flow rate was maintained constant on cold and hot days (tests number 1 and 3), and then, the experiments were repeated, in which the exchanger flow rate was maintained constant and the duct flow rate was variated (tests number 2 and 4). It should be noted that the duct system is placed at the upper opening of the room on cold days, since the hot air has a low density and will concentrate in the upper part of the room, while, on hot days, the duct system is placed in the lower part, since the cold air of the room has a high density and will concentrate at the lower part of the room.
Moreover, in order to validate the experimental results, an uncertainty analysis is considered for the temperature measurements, since the heat transfer rates (recovered heats) depend mainly on the measurements of the temperatures. Variations of the air temperatures at the inlets and outlets of the cold and hot sides for the tests repeated twice under the same operating conditions and configuration and then for different operating conditions (three) are considered (which gives a total of six tests). It was found that the maximum mean temperature difference between the tests is about 0.4 °C, 1.8% relative difference, suggesting that the temperature variation is well-reproduced by repeating the same test and conserving the same operating conditions. On the other hand, the maximum error of positioning thermocouples in the air streams is 0.5 °C. Then, with an average measured temperature of 22 °C, the maximum relative error due to the present method of thermocouple positioning does not exceed 2.3%.
Finally, with 1.8% repeatability and 2.3% precision error of the temperature measurements, the uncertainty gives 97.1% confidence in the temperature measurements.
In the four experiments, for each couple of air speed at the exchanger outlet and downstream, four readings of the temperatures at the different locations are recorded to allow estimating the different heat rates and mass flow rates given by Equations (1)–(4). The results of the four experiments, as well as the corresponding discussions, are presented in the next section.
4. Economic and Environmental Study
The suggested heat recovery setup for all-air HVAC systems was evaluated economically and environmentally for different conditions to estimate the money savings, payback period, and carbon dioxide (CO2) reduction quantity.
In order to perform a comprehensive simplified study, the minimum and maximum amount of heat recovered are considered in the study, i.e., 60 W and 196 W, respectively. Then, the study considered the intermediate levels of performance when the system recovered 105 W and 140 W from the exhausted air.
To proceed, the amount of heat energy recovered per month by this setup
is calculated with the equation below:
where
is the heat recovered, and
and
are, respectively, the number of running hours per day and number of days per month. As the system can run for different numbers of hours, the study will be a function of this parameter.
Eventually, the amount of electric energy
saved per month is estimated by considering a 75% energy conversion efficiency [
27] of the air conditioner (
) and is represented in
Figure 5a:
Figure 5a shows that the electric energy saved ranges between 2.4 kWh and 190 kWh per month on the basis of the amount of recovered energy and the system running time (hours). On hot days, and when the system is capable of recovering 196 W, the electric energy saved raises from 8 kWh to 190 kWh when the system runs 2 and 24 h, respectively, while it increases from 2.4 kWh to 58 kWh when the system is able to recover 60 W (on cold days). This means that the system is more cost-effective on hot days compared to cold days and capable of economizing more electric energy, which will directly affect the amount of saved money, payback period, and amount of carbon dioxide gases reduced.
Additionally, the amount of money saved monthly
is determined by Equation (7) and shown in
Figure 5b.
where
is the cost per kilowatt/hour. The cost per kilowatt/hour in Lebanon is subjected to progressive tax [
22] and can be estimated based on Equation (8).
Figure 5b shows that the system is able to economize about 6
$/month on hot days at full run and about 2
$/month on cold days (approximately at Q
rec = 105 W). It should be noted that the system runs at a low air mass flow rate, and as it increases, the money saved increases. This means that, if the system is allowed to run at a higher flow rate, more money is expected to be saved.
To evaluate the recovery system’s payback period, the overall system cost must be estimated. The system is composed of a heat exchanger, duct, and pipes, with a total cost of $100, including the installation cost and welding.
Based on previous comments, the payback period
equation is as follows:
Figure 6a shows the payback period. It shows that the duration of the payback is highly affected by the number of system running hours. It highly descends as a function of the number of hours. The system is able to pay for itself after 1.5 years when it is used 24 h per day on hot days at 196-W thermal recovery, whereas it requires at least 6.3 years when it is used on cold days at a 60-W thermal recovery rate. It should be noticed that, if the system is utilized for about 8 h per day, the payback period of the system will be less than 6 years if 196 W is recovered by the system. This duration is less than the lifespan of an air conditioner.
Lastly, as for the environmental concern, the quantity of CO
2 gas reduced
is calculated using Equation (10) and represented in
Figure 6b.
where
is the quantity of carbon dioxide produced for one kilowatt/hour of electricity generated. Based on Lebanese studies, the amount of CO
2 generated is 0.47 kg/kWh [
16].
The system can reduce up to 1 ton of carbon dioxide emissions yearly when the system recovers 196 W, which decreases to a 300-kg maximum when it recovers 60 W. However, it should be noted that, in fact, the system reduces more emissions, since this study is based on the electric energy of air conditioners while more electric energy should be generated at an electric power plant to cover the losses through the grid lines.
Since hot/cold air is allowed to move at a low flow rate, the money saved, payback period and CO2 quantity reduced are relatively low. However, those results show a very promising system when run at higher flow rates, allowing for greater energy recovery.