Energy has become the backbone of humanities daily activities. Heating, ventilating, and air conditioning systems (HVAC), which consume around 39% of energy in the residential sector, have turned into an essential constituent for providing fresh air, especially after COVD-19, not only in hospitals but also in any simple construction. Thus, decreasing this percentage or recovering part of the energy lost is an essential issue in today’s energy management scenarios.
1. Introduction
Energy has always been an essential requirement for the existence of all living organisms, as it is essential for growth, movement, maintenance, and creating work. Nowadays, with the rise of technological evolution and the rapid development of applications that need work, the human lifestyle has become more and more energy-dependent, especially in developing urban cities
[1]. For instance, India has witnessed a rapid increase in energy consumption of around 16 times during the last six decades
[2]. The International Energy Agency (IEA) predicted that by 2050, global energy consumption will increase by 50%
[3][4], and buildings will account for the largest source of emissions due to the rapid growth in industries. Likewise, the growing demand for energy is likely to be more intense in growing states due to the growth of new buildings
[5].
Based on the first law of thermodynamics, rising energy demand, global warming, energy shortage, and the necessity of providing fresh air, particularly after COVID 19, makes reducing energy loss a significant challenge
[6]. The reduction of energy loss leads to developing a new strategy that arranges the use of energy, and, most importantly, ensures that power, which would otherwise have been lost, is made use of and provides benefits
[7]. This strategy is called recovering lost energy
[8].
Energy management has been comprehensively studied for almost 40 years. Recently, it has emerged as one of the most challenging issues and popular research topics, where its importance is equal to that of finding a new source of energy
[9]. This is because the percentage of lost energy sometimes surpasses 60%
[10], and, in addition, 72% of all-inclusive primary energy consumption is wasted during energy conversion
[11]. Thus, recovering part of this loss is very beneficial. Lately, in residential and commercial buildings, energy consumption has escalated firmly. The main reason for this was due to the HVAC systems in those buildings
[12][13]. HVAC accounts for almost 50 to 82% of the energy, of which 40% of the world’s overall final energy is spent in buildings
[14]. While in non-industrial buildings, HVAC is responsible for around 18–35% of the total energy consumption
[15], in commercial buildings HVAC accounts for approximately 30% of energy consumption
[16][17]. In some countries, like Sweden, HVAC is frequently used to reduce radon problems, which makes heat recovery a vital requirement to reduce energy consumption
[18].
The high-energy consumption of HVAC contributes to Energy Management System (EMS) becoming a fundamental issue for improving efficiency and providing significant energy savings in construction, particularly in relation to hospitals due to their utility for removing contaminated air
[19]. Nowadays, EMS has become a primary concern in building projects, with many types of research completed on the BEMS (Building Energy Management System) over the last decade
[20]. In addition, statistical results indicate that the effect of savings of BEMS raises from 11.39% to 16.22% yearly. This is due to the effort of continuous research which has led to improving this area
[21]. Despite the improvements and high interest in research in BEMS, it has been estimated that 90% of HVAC systems do not operate optimally
[21]. This demonstrates the necessity for developing systems to be more effective and, above all, for systems to operate at a lower cost in order to ensure their rapid spread
[22].
2. Energy Recovery Systems
The chart, seen in Figure 1, divided energy management in HVAC into the following categories:
Figure 1. Energy management classified into categories.
-
Outside use, which means that the lost heat that is captured is used for external benefits. In other words, the saved energy is used for external systems, like generating electricity, in our case.
-
TEG was mainly used for heating water by air-water HE
[23]. Using the lost energy for outside use is not only necessary for the heat recovery concept, but also enhances the efficiency of the HVAC system
[24].
-
Inside use, which means air saved energy is used in the HVAC system that is equipped with the energy recovery device in order to improve the efficiency and heat/cool the supply. This section is divided into:
- i.
-
Heat Recovery: energy transferred in this case is just sensible heat; this energy can be transferred by different types of heat devices that will be discussed in detail in the upcoming sections.
- ii.
-
Mathematical Control which is related to the controlling and predicting system, such as a smart system, with which a positive impact on saving energy has been revealed.
- iii.
-
Energy Recovery or total energy recovery, which is described as total because it transfers both the latent and sensible heat; this advantage increases the efficiency and competency of the enthalpy devices in energy recovery.
2.1. Energy Recovery for Outside Use
In the following section, the saved energy is used for external systems, such as heating domestic water, or using the captured lost heat to generate electricity. This process shows remarkable results, as it is considered a free source of energy.
2.1.1. Air-Water HE
The concept of heating domestic water from the recovered lost heat is achieved by capturing the rejected heat from the condenser by using a heat exchanger, where the cold water is heated by the warm flow that is produced from the condenser. Table 1 summarizes the methodology and results of the main investigated papers that were related to energy recovery using an air-water heat exchanger.
Table 1. Summary of the investigated papers related to heating water for domestic use from the lost heat.
From Table 1, it can be observed that the results obtained in the air-water HE system were considerable, and it is worthwhile heating domestic water from the lost heat. This system is efficient due to:
-
The high heat capacity of water which allows it to conduct heat at a rate which is about 25 times faster than air. Therefore, water is considered to be more efficient than air.
-
The increase of the outlet temperature of the water as the load increases. This indicates that applying air to the water heat exchanger in buildings that have a high use of HVAC is very efficient.
2.1.2. Thermoelectric Generator TEG
This system is also called a Seebeck generator. It is a solid-state device, which transforms the difference of temperature on its opposite sides into electricity following the phenomenon of the Seebeck effect. TEG is a considerable technology that recovers the lost heat in various applications
[28].
Table 2 represents the methodology and results of the main investigated papers that were related to energy recovery using a Thermoelectric Generator TEG.
Table 2. Summary of the investigated papers related to generating electricity from the lost heat.
As a conclusion drawn from Table 2, it is observed that:
-
TEG has low efficiency due to its design.
-
However, in some cases, it produces significant results due to higher gradient temperature.
-
When the temperature difference increases, the value of generating electricity increases.
-
TEG shows a positive effect in HVAC but some improvements should be made in order to increase its efficiency.
2.2. Energy Recovery for Inside Use
In this section, employing saved energy for internal use through previous published paper is discussed.
2.2.1. Heat Recovery
Air-Air HE
A method of capturing heat loss from hot air enclosures, the air-air HE is a type of system designed for exchanging heat. This, however, occurs in a passive way, whereby the design enhances the heat transfer.
Table 3 shows the methodology and results of the papers that studied air-air energy recovery devices (HPHE, THE and PHP/OHP, Run around coil)
[32].
Table 3. Summary of investigated papers related to air-air energy recovery devices.
Table 3 revealed that:
-
Air-to-air HE decreases considerably the energy consumption, but there are some limitations according to its characteristics and specific configuration.
-
Many factors affect efficiency such as working fluid, size, and climate.
-
It is highly recommended to combine other systems, such as PCM, in order to overcome its overheating problem.
-
It is noticed that OHP in continental climates can save higher energy.
-
The efficiency of THE in winter is higher than in summer. It is recommended to perform a comparative study for each of THE and HPHE in different climates.
-
Papers on integrating run-around coil with HVAC were not available as much as other heat recovery devices. However, it is concluded that this technology is noticed to be effective in cold climates
Earth-Air HE
EAHE is considered an encouraging technology, which can efficiently decrease the load of cooling/heating of a building by warming up the air in the wintertime and the same in summer. Table 4 shows the methodology and results of the papers that studied energy recovery from EAHE.
Table 4. Summary of the investigated papers related to energy recovery from earth to air HE.
It is observed that:
-
EAHE is effective in severely cold/hot weather, when the temperature is high, which results in a higher temperature gradient which means higher heat transfer.
-
It is noticed that the EAHE is most efficient in hot dry climates weather.
-
EAHE depends on some parameters to achieve optimum energy saving, such as the ground, soil properties, depth, climate, working fluid, geometry, and material of the pipes used. As such, location and climate should be taken into consideration.
2.2.2. Mathematical Control
Controlling and smart systems are great inventions that helped in many fields. These systems do not just facilitate the process, but also save energy, as will be shown in the methodology and results of the papers that studied the effect of mathematical control on HVAC systems in Table 5.
Table 5. Summary of the investigated papers related to mathematical control effect on HVAC systems.
Mathematical control is related to controlling and predicting systems, such as the smart system. These systems have been revealed as having a positive impact on saving energy, as shown in
Table 5, where it is noted that the model predictive control (MPC), building automation systems (BAS), estimation models, and automatic smart systems have acquired high attention in HVAC for their ease of use. However, these systems require a high cost and level of accuracy in their construction
[59][60][61][62][63][64]. From
Table 5, it is noticed that the model accuracy is acceptable. In addition, the model showed a significant positive effect on saving energy. Further research that involves a smart system in HR devices is recommended.
2.2.3. Total Energy Recovery
Enthalpy Wheel
An enthalpy wheel heat exchanger is one of the energy recovery devices that transfers sensible and latent heat. This criterion increases the efficiency of RW and helps in decreasing the moisture in the air. Table 6 shows the methodology and results of the papers that are related to the enthalpy wheel heat exchanger in HVAC.
Table 6. Summary of the investigated papers related to the enthalpy wheel effect on HVAC systems.
The results of Table 6 show that:
-
EW HE is effective in HVAC applications, due to its high efficiency and ability to transfer latent and sensible energy, which can be saved.
-
In addition, it was noticed that energy recovery increases when the temperature of fresh air and moisture content increases; consequently, the chance of recovering heat using EW rises.
-
It is also noticed that the efficiency of RW in hot and humid climates increases.
-
As such, it is recommended to study the effect of RW in a humid and cold climate. It is logical that the results will be positive where humidity has a positive effect on RW, as it recovers latent heat.
PCM
PCM is a material that releases and absorbs while phases change. This process provides heating and cooling by melting and solidification. PCM offers a storage ability, which makes it useful in many applications. Table 7 shows the methodology and results of the papers that are related to the PCM effect on the HVAC system.
Table 7. Summary of the investigated papers related to PCM effect on HVAC systems.
The studies in Table 7 indicate that:
-
PCM possesses a significant effect in terms of energy-saving and storage.
-
The PCM+EAHE system is encouraging, especially due to its enhancement in swing temperature reduction.
-
PCM has a high cost and some limitations, such as its low thermal conductivity
[73][74].
-
As such, it is recommended that PCM is used to aid the storage system in order to store the excess heat and discharge it gradually, such as PCM combined with HP, THE, or EAHE where PCM offers the storage which helps in extending the lifetime of the system.
Fixed Plate
The fixed plate uses metallic plates to transfer the heat between fluids. Its novelty is that it can expose the fluid to a greater surface area, as fluids blowout over different plates. This helps in increasing the heat transfer rate for the compacted size. FPHE can transfer latent energy as well as sensible energy, especially when combined with a liquid desiccant dehumidification system (LDDS)
[75].
Table 8 shows the methodology and results of the papers that are related to the fixed plate membrane energy recovery effect on the HVAC system.
Table 8. Summary of the investigated papers related to the fixed plate effect on HVAC systems.
Table 8 exhibits the following conclusions:
-
FPHE is a promising method to save energy in HVAC applications.
-
It is noticed that the intake air conditions affect the performance of the system, such as increasing the airflow rate and RH, because fixed plate HE transfers latent and sensible energy.
-
The shape of the fixed plate has a significant effect on the amount of recovered energy, whereby results show that the L shape fixed plate shows higher recovered energy.
-
In addition, it is noticed that the amount of recovered energy is higher in humid climates than in dry and moderate climate environments.
3. Effect of Weather Conditions
Based on the above-investigated papers, it is observed that climate has a considerable effect on the performance of each device
[80][81]. As such, in this section, energy recovery devices will be classified according to their best climate conditions based on previous research.
All of the papers agreed that the cost savings and efficiency of the systems are higher in winter than in summer
[82]. Thus, heating is more efficient than cooling for the same system
[83][84][85]. In addition, Wu et al.
[86] indicated that when relative humidity exceeds 70% in hot and humid climates, latent heat becomes a significant constraint. Therefore, energy recovery devices are suitable for such climates due to their ability to transfer both latent and sensible heat systems
[36][83]. As such, rotary wheel and fixed plates can be used in cold humid climates, but there is a freezing problem at very low temperatures. In addition, for the fixed plate, the condensation builds up problems, limiting its implementation in severely cold weather. In winter, it is found that enthalpy devices recover over 25% more energy than sensible heat devices. Whereas, in summer, the energy recovery device recovers about three times as much energy as the sensible heat devices
[87].
Ground heat pump (GHP) systems have been widely implemented in cold climates. Results show that GHP in cold climate regions is slightly improved, where energy savings are around 7.2% and energy cost savings are on average 6.1%
[88]. A horizontal air-ground heat exchanger (HAGHE) system, which is a type of geothermal energy, reduces the consumption of energy in all seasons. These systems show high effectiveness in various climates (hot and humid, cold climate, tropical climate, Mediterranean climate, moderate climate, etc.)
[89]. However, they perform optimally in a hot-arid climate, where a reduction of 66% in the gradient temperature between the highest and lowest daily temperatures occurred over the year
[52]. Whereas, in the cold climate, the reduction was lower
[53]. Thus, EAHE performs in a hot-dry climate better than in a cold climate. These results demonstrate that GHP and EAHE are applicable in hot dry-climate regions.
An experiment was completed in India which showed that heat pipes (HP) saved maximum energy in warm and humid, or hot and dry climates
[2]. The results also revealed that wraparound heat pipes, HPs wrapped around a cooling coil, are applicable for hot and humid climates
[38]. Thermosiphon is also recommended to be used in a subtropical climate (hot and humid summers, cold winters)
[90]. As such, HP is mainly reliable in a hot climate. However, there is a lack of research about HP in cold climates. As such, it is recommended that an experimental study that compares the use of HP in a cold dry climate and cold humid climate is undertaken.
OHPs are most efficient and cost-effective in continental climatic conditions (hot summers and cold winters). G. Mahajan et al.
[40] estimated that when OHP was involved in heat recovery ventilator (HRV) systems, more than $2500 were saved yearly in cities with continental climatic conditions. OHP-HRV offers a total average reduction of 16% in energy consumption annually, so it shows a high potential for dropping energy consumption, as well as reducing the operating costs. In addition, it was shown that OHP is suitable for a sub-humid tropical climate, where the WHR of the heating mode exceeds 80% of the total annual WHR. Thus, OHP is reliable in a hot humid climate. It is recommended that OHP efficiency in a hot dry climate is studied and compared to a hot humid climate.
PCM revealed good results for both heating and cooling
[91]. Yet, its optimal performance depends on the chosen material. For example, in a cold climate, the material’s melting point should be around 26 °C, while in hot climates, a 20 °C melting point leads to better energy savings
[92]. Research revealed that PCM is reliable in a tropical climate, like Chennai
[93][94]. Thus, PCM is most efficient in a non-arid climate.
Figure 2 proposes a classification of the devices according to their best climatic condition, where each device achieves its optimal effectiveness. For example, RW showed high effectiveness in all climates, but its efficiency in humid climates is better than in arid climates, and thus it is placed in humid climate. This is similar for EAHE, which shows significant results in a hot dry climate, as mentioned in this section. The same procedure goes for the other devices that offer their optimum performance according to the climate.
Figure 2. Heat recovery devices according to the climate.