Adding Rethink to the mantra of Reduce, Reuse and Recover, Shahid Akhtar Nazeer Ahmed demonstrates how to effectively utilise FAHU exhaust air to enhance the efficiency of air conditioning systems.

The green movement globally has focused on Reduce, Reuse and Recover. The HVAC industry has also contributed by creating products and following practices based on these guidelines. This green initiative is essential today, as the sector accounts for 50% of the power consumption in the Middle East region.

Introduction
The air conditioning cycle is based on recirculation of refrigerant, water and air.

The focus in the air conditioning industry has been to “Reduce” the power consumption in each component to achieve best efficiency of the system, as well as to “Recover” energy by introducing new components in the air conditioning system, which in turn increases system efficiency.

In the last decade, introduction of new and better control of the components has allowed a shift of focus in the industry. Moreover, there is continuous encouragement from the Dubai government and Dubai Silicon Oasis higher management for smart cities, green initiatives and innovative smart solutions, which are especially applicable to the Middle East region and climatic conditions.

One such system, “FAHU Exhaust air utilisation”, as implemented and tested at Dubai Silicon Oasis, has shown considerable reduction in power consumption. If incorporated during the HVAC initial design stage of a project, it can have a notable energy saving of around six to 10% (Refer to calculations). This is a simple system of reusing the exhaust air from fresh Air Handling Units (FAHUs) by supplying it to AC condensers. For better understanding, a brief description of the conventional FAHU (A.1) as used in Air Conditioning systems and heat rejection in conventional DX systems (A.2) have been included.

1. Conventional FAHU and air-cooled systems

A.1. Conventional Fresh Air Handling System (FAHU): The basic function of a FAHU is to supply outside fresh air and maintain Indoor Air Quality. The external fresh air is filtered and cooled using a cooling coil and then supplied to the space. In Dubai, the air entering the cooling coil is at around a temperature of 46.1 DB degree C / 30% RH (at peak), and the air leaving the cooling coil is at a temperature of 21 DB degree C / 70% RH, leading to a certain cooling coil capacity requirement.

To increase the efficiency of this system, the exhaust air from kitchens, toilets, and sometimes the AC space itself is used, which is usually at 24 degree C (in some cases as low as 22 degree C). The exhaust air is passed through a heat recovery system in which the heat present in the outside fresh air is partially transferred to the exhaust air (heat from higher temperature air is transferred to the lower temperature air), leading to the air entering the cooling coil temperature to be around 33 DB degree C / 55% RH. Thus, the energy input required to cool the fresh air is reduced for the same air leaving the cooling coil conditions.

The same amount of treated fresh air is supplied as in the earlier case, but with a cooling coil of lower capacity, thereby reducing the running cost of the equipment. The return air which has gained heat from the fresh air is now exhausted into the atmosphere. The air that is exhausted into the surrounding atmosphere is usually at a temperature of 36-39 degree C (In some cases it drops to as low as 32 degree C), which is lower than the ambient temperature of 46.1 degree C. And this cool air is exhausted into the atmosphere without further recovery.

Refer to Figure 1, which shows the components and air flow directions of a conventional fresh Air Handling Unit with heat recovery wheel.

It needs to be noted that the effective utilisation of exhausted air can be carried out only by selecting the right air conditioning equipment and its performance in the region.

A.2. Heat rejection in conventional air-cooled system: The air conditioning system captures the heat energy from within the AC spaces and rejects it into the environment using air or water as the medium. Due to the limited clean water source and high cost of desalination in the Middle East region, typically, the choice of heat rejection medium is air. The lesser the surrounding ambient air temperature, the more the efficiency and vice versa. Heat rejection by air is achieved using outside ambient air and passing it through a finned coil condenser.

Condensers are rated in terms of total heat rejection (THR), which is the energy absorbed at the evaporator plus the work input to the compressor. The THR is also the product of the refrigerant mass flow and the enthalpy difference between the refrigerant vapour entering and the refrigerant liquid leaving the condenser coil. Therefore, at high ambient temperature available for condensers, more power will be required by the compressor to raise the refrigerant temperature.

Refer to Figure 2 and the charts showing the variation in cooling capacities and power consumption of Ducted Split Unit SADE 30 / SBHC 17 with the varying outdoor temperatures.

If the cooling capacity of the above-mentioned model is considered at an ambient temperature of 115 degree F, the cooling capacity of the system would be 25.3 TMBH with 1300 CFM airflow at an on coil temperature of 80.0 DB degree F / 67.0 WB degree F. In order to reject this amount of heat into the atmosphere, the system requires 2.7 kW of electrical power input.

Considering the same equipment at an outdoor temperature of 105 degree F, the cooling capacity of the system will be 27.0 TMBH. The net heat rejection into the atmosphere increases by 6.7%. This increase in heat rejection means an improved cooling performance of the air conditioning equipment. Additionally, the system consumes only 2.5 kW of electrical power, which is 7.4% lower than the performance at 115 degree F. If the outdoor temperature is 100 degree F, the enhancement in cooling capacity and the reduction in power consumption is around 11%.

1. Effective exhaust air utilisation from the FAHU

As mentioned in A.1, the air that is exhausted into the surrounding atmosphere is usually at a temperature of 36-39 degree C, which is lower than the ambient temperature of 46.1 degree C. This exhaust air can be further utilised by supplying it directly to the air-cooled condensing units as mentioned in A.2. The exhaust air gets mixed with the ambient air, leading to air with a temperature of around 38-39 degree C near the condensers, which then passes over the condensing coils. When the condensing coil gets air at a lower temperature than the ambient temperature, the total heat rejection into the atmosphere by the split system increases, thereby increasing the cooling capacity of the split system. At the same time, this phenomenon reduces the power required to achieve the cooling capacity.

Figure 3 (opposite page) shows a model demonstration of the proposed system, ie, supplying the exhaust air to the air-cooled condensing units. In addition to this, the system allows selection of FAHU condensing units or chiller at a lower design temperature, if considered at the earlier design stage.

The most significant benefits of the proposed system are its simplicity and almost negligible extra cost. Typically, the FAHU and normal DX condensers are placed together close to each other at the roof, and all that requires to be added is a guiding duct, as indicated in Figure 3, which can lead the comparatively cool exhaust air from the FAHU to DX condensers. Moreover, when a building is not completely occupied or at part load, and the AC temperatures drop to less than design, then the exhaust from the FAHUs are at further lower temperatures, causing excess waste of energy. (We can notice the same on most of the building roofs.) The above-proposed system can even control the additional wasted energy.

Conclusion
The smart city ecosystem, green building regulation and the growing requirement of maintaining Indoor Air Quality makes it imperative for every building to be installed with a fresh air management system. In light of this, the proposed solution is a combination of an effective exhaust recovery system and the use of high-efficiency products, which will have an enormous impact on the energy consumption in regions with high temperatures.

Clearly, the new “R” has gained preference among energy and waste management experts, who are convinced of the efficacy of incorporating ‘RETHINK’ into the paradigm.

AkhtarNazeer Ahmed is Associate Manager – Mechanical, Planning & Design Department – Engineering Management, Dubai Silicon Oasis Authority