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Air Conditioning Options For Buildings

Central air conditioning (AC) systems using refrigerant R410A can be classified on the basis of mode of transfer of cooling effect as:

•-Chilled air
•-Chilled water
-Refrigerant (variable refrigerant flow)

The oldest AC plants are the chilled air systems where the air is chilled through direct expansion of refrigerant and the chilled air is distributed across the cooling spaces. The advantage of this system is that it handles only air in the secondary cooling side and involves low risk of loss of refrigerant. The major disadvantage of this system is that air has low density and handling air involves large ducts which occupy space, results in operational noise and vibrations due to fan operation, consumes high electrical power for transfer of air to the conditioned spaces and there are losses of both air and cooling effect during the transfer.

To overcome this deficiency of handling large volume of chilled air circulation, large volume of space occupied by the ducting, chilled water circulation systems were introduced. The chilled water is pressurised demineralised water is circulated between the evaporator (chiller) and the load. Further down, the cooling effect is transferred through the air cooling systems. The chilled water system reduces the volume occupied by the AC system considerably and is transferrable between buildings, across multiple storeys. The cooling effect is transferred from the chilled water lines to the conditioned spaces through air handling units (AHUs) or fan coil units (FCUs) for individual rooms.

The most modern system is to have a large condenser consisting of high pressure liquid refrigerant, transfer it to the vicinity of the required conditioned spaces and then expand the liquid to vapour in an evaporator located in the conditioned space as a split system. The compressor, condenser and the cooling system like cooling tower, are all located in a single outdoor location and can be called as an outdoor unit. The evaporator is located indoor very near the area required to be air conditioned. This system is called as VRF system.

In VRF systems, since the high pressure from the condenser is being transferred, there is no requirement for insulation of the piping and the cooling effect is realised only on expansion after the expansion valve in the evaporator. This system will avoid the energy losses (cooling energy loss) due to insulation of the chilled water pipelines between the chillers and the AHUs. Also, the AHU losses are avoided. Therefore from the angle of energy efficiency this system is more efficient than the other two systems described above.

Typical advantages of VRF systems are zoning flexibility implying that independent temperature control for different rooms/spaces. Capacity controllability, ease of retrofitting, low installation cost, minimization of ducting space. It is possible to have refrigerant piping of as much as 100 m with multi-splits in ceiling cassette configurations and concealed ducting adding to the aesthetics of the building design. In zones which are inadequately cooled by conventional AC systems, the VRFs come in handy.

New design advances like the use of variable frequency drives for compressors and cooling tower fans of condensers for energy saving capacity modulation can be usefully employed for capacity control of VRF systems.

The main disadvantage of the VRF systems are a large condenser volume, large volume of refrigerant (refrigerant charge is more than double that of conventional systems), large ducting of refrigerant piping which can be potential sources of refrigerant leaks from the system. Another major disadvantage of the system is the requirement of a separate ventilation system for air change as this system provides only cooling effect and ventilation has to be separately taken care of.

Energy efficiency of AC plants

The indices are both inherent (machine dependent) and installed dependent on both machine and its operating environment).

Three inherent indices of energy efficiency are:

•-SP (Specific electric power) (kW of electric power input per tonne of refrigeration = kW/TR) [1 TR=3.516 kW]
-EER (energy efficiency ratio) = cooling load (kW)/electric input (kW) which is expressed on the basis of p.u., it is generally in the range of 2.3 to 3.1.
-COP (coefficient of performance) is given by the ratio 3.516/SP

The above three are interrelated. The installed energy efficiency index is the SEC (specific energy consumption) in kWh/m2/year or in kWh/TR/year. As per the Energy Conservation Building Code of India, the energy efficiency of buildings must be within 120 kWh/m2/year for AC buildings and 25-40 kWh/m2/year for non AC buildings. The AC contribution can be taken as 80-90 kWh/m2/ year. The AC power is benchmarked at 25 W/m2. To compute the specific energy (kWh/TR/year) from specific power rating (kW/TR), the on time to total time ratio is ratio of the period under which the AC plant is in operation and drawing full active power to the total time period under consideration (24hours/day; 720 hours/month or 8760 hours/year). This is a temperature dependent factor and is designed to be around 0.1 to 0.3 for the ambient temperature of 33-34ºC and is reduced to zero when the ambient temperature coincides with conditioned air temperature.

Table 1 shows the cooling load per unit area for different sizes of AC systems. Table 2 indicates Electrical load for cooling per unit area (floor space) for conventional compression air conditioning and Table 3 shows the annual energy consumption for cooling per unit area (floor space) for conventional compression air conditioning. In the case of variable refrigerant flow systems (VRF or VRV) and energy saving of around 10% can be expected to account for reduced energy losses of cooling systems (cold air ducts or cold chilled water pipelines) since liquid refrigerant at high pressure is transferred and expanded to low pressure at the load end. Beside the reduced losses due to dispersion losses of the cold air and water piping, the energy savings are resulting from capacity control. Most ACs do not operate at 100% of the load but at 30-70% load for which efficient capacity control is called for. The VRF systems are able to modulate their refrigerant output to suit the load requirement and with VFDs energy savings can be achieved at part loads.

Economics of AC plants

Table 4 gives the capital cost for different systems. The high cost of the central and VRV systems is because of the ducting required to be drawn whereas for split systems only electrical supply needs to be provided and the ducting lengths are quite small for each individual unit. Though the cost on cooling load basis (TR) are significantly different the cost per m2 of floor space cooled are showing less variations. The pay back period per m2 of floor space cooled is around 6-7 years for VRV systems as compared to central ACs and 9-11 years as compared to split ACs.

Applicability of VRFs for multi room cooling spaces (hotels, hostels and training centres)

Typically in multi storeyed, multi room cooling spaces (hostels, hotels and training centres) contain the following types of spaces to be air conditioned:

•Common spaces like:

-•Lounges • Dining Halls
-Lecture/Meeting halls
-Banquet halls
-Double rooms

The ratio of the living room spaces to the common spaces is around 70-80% of the total conditioned space. For central areas, central ACs or VRF central systems can be considered. For individual rooms split ACs can be considered. In multi storeyed systems with several elevations, the space occupied by chilled air ducting becomes quite considerable and it is better to go in for fan coil units installed in individual rooms with chilled water circulation to the rooms. An alternative to the conventional chilled water system is the VRF system where the refrigerant can be conveyed at high pressure to the cooling area and expanded therein.

The choice between (a) central systems for the whole building, (b) split systems for individual rooms and conventional central systems for common cooling spaces, and (c) VRFs for the entire building, in most cases depends on the capital cost made available at the time of building construction. Where there are capital cost constraints, users tend to go in for option (b) in two phases. Where aesthetics are not compromised, then VRFs are ideally suited. In the case of retrofitting of old buildings which do not have provision for duct work or ducts, VRFs are the ideal choice. VRFs also address the cases where conventional ACs are unable to provide the required level of cooling in the rooms or areas where cooling is required. VRFs overcome the poor controllability of AHUs of conventional ACs. By coupling the VRFs with heat recovery wheels for recovery of cooling effect, there could be good energy saving up to 70%.

Concluding remarks

-•For multi storeyed, multi room cooling spaces (hostels, hotels and training centres) the air conditioning choice is between (a) central systems for the whole building, (b) split systems for individual rooms and conventional central systems for common cooling spaces, or (c) VRFs for the entire building.
-In most cases the choice depends on the capital cost made available at the time of building construction. High capital cost is one of the main deterrents to this technology as a result of which the market penetration is hardly 1 in India and 2-3 % in Europe and Japan.
-Energy wise VRFs give a saving of around 10 % but the savings are offset by high capital cost which is Rs 1.2 lakhs/TR as compared to Rs 0.40 lakhs/TR for split systems and Rs 0.75 lakhs/TR. However, if one goes by the cost per unit floor space the costs are almost comparable (Rs 0.045 to 0.054 lakhs/m2).


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