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High Efficiency Air Conditioner


1. Introduction
According to a survey of 300 buildings conducted by the Energy Conservation Center of Ho Chi Minh, it appears that 52 percent of the total electricity is consumed by HVAC systems in Vietnam. Therefore it is a necessity to install, operate and maintain efficient systems to make substantial savings.

2. How to get an energy efficient AC system?
a. High COP

The coefficient of performance or COP (sometimes CP) is a ratio of heating or cooling provided to electrical energy consumed. Higher COPs equate to lower operating costs.

COP = refrigerant output/power input (kW/ kW)(1)

The COP may exceed 1, because it is a ratio of output:loss. COP is highly dependent on operating conditions, especially absolute temperature and relative temperature between sink and system, and is often graphed or averaged against expected conditions.
The National Technical Code QCVN 09:2013/BXD is specifying a minimum COP level for all machine types and cooling power.

b. Inverter systems
What is an Inverter system?
An inverter system allows to optimize the operation of the compressor motor by exactly matching the cooling demand then reducing fluctuating temperatures around the set point .
By contrast, conventional air conditioners regulate temperature in a binary manner either through the compressor working at its maximum capacity or completely switched off.
Inverter-equipped air conditioners are equipped with a variable-frequency drive that smoothly control the speed of the compressor to match the demand and reduce fluctuations.

How does it work?
The amount of cooling or heating required by an air conditioning unit varies depending on the outdoor temperature and the amount of heat in the room.
When the cooling or heating capacity needs to be increased, the compressor will operate at a high speed and will increase the amount of refrigerant flow. Conversely, during moderate outside temperatures for example, when the cooling and heating capacity needs to be decreased, the compressor will operate at a low speed and will decrease the amount of refrigerant flow.
When the inverter air conditioning is switched on, the compressor operates at a high speed in order to cool or heat the room quickly. As the room temperature approaches the set temperature, the compressor slows down, maintaining a constant temperature and saving energy.
Any sudden fluctuation in the room temperature, will be sensed and instantly adjusted to bring the room temperature back to the set point.
Inverter air conditioners use between 30-50% less electric to operate (2)

Variable speed technology can be implemented in many different HVACR systems, such as packaged or split air-conditioning units, rooftop units, chillers, precision, Variable Refrigerant Flow  and condensing units.

c. Variable Refrigerant/Variable Refrigerant Flow systems
What is a VRV/VRF system?
Variable Refrigerant Volume or Variable Refrigerant Flow (depending on the manufacturer) has been around for about a quarter of a century. It originated in Japan around 1977 and is now widely used in the world. A large outdoor unit (condenser / compressor) serves multiple indoor units. Each indoor unit is equipped with an LEV (electronic liquid expansion value) to control its refrigerant supply to match the demand of the space it serves. This electronically controlled valve system allows to supply refrigerant only where and when needed.

Every indoor unit is provided with indoor temperature sensor which controls the electronic expansion valve on the refrigerant line.
The quantum of flow of refrigerant through the indoor unit is regulated to meet the set indoor temperature requirement.Variations of the refrigerant flow requirement  are achieved by modulating and optimising the compressor’s activity. This solution helps reducing the power consumption drastically.The speed of condenser fan is also regulated following the actual demand which also reduces further the final power demand.

3. References
(1) The National Technical Code QCVN 09:2013
(3) A survey of 300 buildings conducted by the Energy Conservation Center of Ho Chi Minh

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