Using the Natural Environment to Manage Heat Loads - The Case for Free Cooling
The Air Conditioning Solution
Globally, the installation of air conditioning has been proven to have benefits in relation to health, improved learning, productivity and prosperity. Previous prolonged spells of hot weather during the summer months in Europe have seen many cardiovascular health issues arise whose impact has been lessened due to the wider use of indoor comfort cooling. Air conditioning has also opened the tropical areas of the world for development where day-time temperatures previously inhibited productivity, being too hot to work. Studies have also been conducted on the impact of air conditioning on education. Students living and studying in cooler air-conditioned environments achieved better grades than those in living in non-air-conditioned environments.
Air Conditioning, and its related technology – Refrigeration - is now used in all walks of life. Offices, factories, homes, hospitals, residential care homes, hotels, student accommodation and even prisons amongst others are exploiting the benefit of comfort cooling to promote health and well-being. It is estimated that there are currently 1.6 billion air conditioning units in use globally, with a further 1 billion due to be installed in the next 10 years (2/3rd growth).
Whilst air conditioning has many benefits for our daily lives, there are some issues that require management.
Issues with Air Conditioning
Air conditioning is also known as mechanical cooling because it uses power from the local electricity supply to drive compressors and fans that are part of the refrigerant vapour-compression cycle. During periods of warm weather and peak demand, (Midday to early evening), the load on the local electricity supply will be great. A higher load on the electricity supply network will require generating plants to work at high capacity. At other times of the day, this capacity stands idle which is a cost to the generation supply company. The need for temporary increases in generating capacity can lead to investment in cheaper, easier to maintain, less environmentally friendly, fossil fuel powered generation plants to top-up supply capability. Using fossil fuel as a source of electricity creates greenhouse gasses causing global warming which starts a vicious circle of increasing global temperatures and the need for comfort cooling.
Refrigerants are known to cause the greenhouse effect on the planet and each refrigerant has a stated Global Warming Potential (GWP). The GWP is a ration of the number of litres of CO2 are equivalent to a leak of 1 litre of refrigerant. In the case of refrigerant R12, a leak of one litre has the same global warming effect as 10,900 litres of CO2. With the F-Gas directive, it is now no longer possible to use refrigerants with a high GWP in Europe, however, refrigerant leaks in air conditioning systems still remain an issue today.
Another consideration of the use of air conditioning is the efficiency of the system. Inexpensive small systems tend to have lower efficiency levels which increase electricity demand. The peaks and troughs in demand exacerbate the issue of variable demand on the local electricity supply and the provision of inexpensive temporary generation capacity.
Where does Free Cooling Step In?
When it comes to humans, thermal comfort is an important factor in our well-being. There are other however other processes and equipment that we use in daily life where operating temperature limits can be much higher than what would be acceptable for us.
Information and Communication Technology (ICT) equipment dissipate energy in the form of what is known as Sensible Heat. When an electrical or electronic circuit is working, there are losses due to the resistance of the components in the circuit. The greater the power flowing through the circuit the greater the temperature rise of the components.
Sensible heat is a dry heat which can be removed by convection. If the heat load is small or the circuit is low power, natural convection over the components may be sufficient to draw away the heat generated by the dissipation of energy. With larger heat loads, such as high capacity ICT equipment, forced cooling using a fan may be required. The amount of airflow required for cooling is calculated using a simple formula.
The formula considers the power being dissipated with the difference between the ambient air temperature and the maximum allowable operating temperature of the components in the equipment.
When can Free Cooling be used?
In order for cooling to happen, the ambient air must be cooler than the equipment it will be cooling.
For example, in the case of a small electronic enclosure being force cooled with a compact fan:
- If the ambient air temperature is 30oC and the air inside the electronic enclosure is 35oC, cooling will take place.
- If the ambient air temperature is 35oC and the air inside the electronic enclosure is 35oC, cooling will not take place and the temperature inside the enclosure will start to increase
According to the Met Office, the prolonged spell of warm weather that was Summer 2018, the average temperature was just under 20oC in the south of the UK with maxima of just over 30oC on a few days in some parts - see https://www.metoffice.gov.uk/news/releases/2018/end-of-summer-stats.
ICT equipment will have a stated operating temperature range, the upper limit of which can be in excess of +50oC. Equipment with an upper operating temperature limit of +50oC would mean that the ambient temperature in the south of the UK during the summer of 2018 would have been sufficient to ensure cooling, even on the hottest days.
The amount of cooling required will depend on the amount of power dissipated by the equipment as heat and the difference between ambient and maximum operating temperatures. Using this information, the required airflow can be calculated. As the ambient temperature rises, the volume flow required for cooling will rise.
Where is Free Cooling currently being used?
Ubiquitous in our daily lives but seldom noticed, StreetSide cabinets are one example of equipment using free cooling. Originally designed for telecom use, they now house electronic equipment for the delivery of high-speed broadband internet services. Air is drawn in via ventilation grilles passing through a labyrinth for protection against ingress of moisture and filtration for protection against ingress of dust. As internet traffic increases, the temperature of the equipment increases and the temperature of the air inside of the cabinet rises. Temperature controlled fans sense the rise in temperature, switch on and modulate fan speed based on a pre-programmed response profile.
The types of fan used in this application can be compact axial for high volume flow against low resistance or backward curved motorised impellers for higher pressure, higher grades of filtration.
Controllers that have multiple sensor inputs can be used to monitor the temperature at specific points within the cabinet and the controller can be set to control based on an average of the two sensor readings, use the highest reading to set the speed of the fans or use the sensor to control specific fans installed within the cabinet.
Free cooling is not limited to StreetSide cabinets, any application whose maximum operating temperature is higher than the maximum ambient temperature of the local weather conditions can use free cooling to maintain reliable performance.
Products from RS Components that can be Installed in a Free Cooling System
High Performance 24V DC axial fans available via RS components include
400 series - (840-5560)
600 series - (878-1050)
8000 series - (749-6966)
4000 series - (055-7701)
High performance 24V DC Diagonal flow fans available from RS Components include
DV4000 series - (826-1165)
DV5000 series - (826-1171)
DV6000 series - (826-1193)
High Performance 24V DC Centrifugal fans available from RS Components include
RG160 series - (825-7901)
RER101 series - https://uk.rs-online.com/web/p/blowers/2508416513/?sra=pmpn
RER175 series - (860-0223)
RER190 series - (920-9206)
CPCXX – DCP Temperature based fan speed controller with open collector PWM output
- The controller includes:
Pre-set temperature profile controls the fan from 15-100% speed
Compatible temperature sensor on 2m lead
RS Components part numbers:
Supply voltage 11-57V DC & Control range 20-40oC RS1(65-1397) - CPCXX2040SC-R
Supply voltage 11-57V DC & Control range 20-40oC RS(165-1398) - CPCXX3555SC-R
CPCXX – A2P Analogue 0-10V input to open collector PWM output fan speed controller
The controller includes:
- Fan monitoring
- Pre-programmed control response
- 0V input = 0% PWM output (signal low)
- 10V input = 100% PWM output (signal high)
RS Components part number:
- Supply voltage 11-57V DC & Control range 0-10V input 0-100%PWM Output, RS (165-1401) – CPCXXVV10UN-R
CECXX – Temperature based fan speed controller with 0-10V DC analogue output
The controller includes
- Fan monitoring
- Pre-set temperature profile controls the fan from 15-100% speed
- Alarm output
- Compatible temperature sensor on a 2m lead
See RS Components part numbers:
- Supply voltage +10V DC with Control range 20-40oC – RS (165-1399) - CECXX2040SC-R
- Supply voltage +10V DC with Control range 20-40oC – RS (165-1400) – CECXX3555SC-R
CGCXX – User Configurable Temperature / 0-10V based fan speed controller – one fan output
Fan controller that combines the features of both the CPC and CEC product ranges with a Microsoft Windows™ based Graphical User Interface (GUI) software package to allow the controller to be configured according to requirements. The controller includes
- Wide operating voltage range (+10V to +57V DC)
- Configurable control input (Thermistor / 0-10V DC analogue signal)
- Configurable output signal (Open collector PWM / 0-10V analogue signal)
- Configurable set-points
- Simple control profile (4 program points)
- Multi-point profile (up to 256 program points)
- Minimum speed cut-off
- Programmable Open Collector Alarm Trigger (Fan fail / Over temperature / Under temperature)
- Maximum fan speed limitation
See RS Components part numbers
- Supply voltage +10V to 55V DC User configurable controller – RS (165-1402) – CGCXX00000
- Controller configuration lead – RS (769-2726) – 210-HAR11887
- Graphical User Interface and instruction manuals (Free to download)
Thermal Management System (TMS) Controller
The Thermal Management System controller is fully configurable and can speed control 4 fans independently based on either a temperature sensor or an analogue 0-10V control signal input. The fans can either be 4-wire open collector PWM control or 4-wire 0-10V analogue control.
See RS Components part numbers
- Development Kit with +12V to 57V DC User configurable controller – RS (769-2732) – TMSB11111-01
- Boxed controller only +12V to 57V DC User configurable controller – RS (769-2722) – TMSB00000-01
- Controller configuration lead – RS (769-2726) – 210-HAR11887
In summary, for information and communication technology, using the local ambient air with some provisions for moisture and particulate protection can provide adequate all-year-round free cooling without the extra power requirements a mechanical cooling system needs.