By improving the power factor in an electrical circuit, capacitors can directly curb CO2 emissions.
It’s a fact. If you connect capacitor banks to your electrical network you save energy and reduce your carbon footprint. Actually, in an electric system that typically powers a commercial building or a facility like a water treatment plant, capacitors can help you cut your carbon emissions by some 100 kilograms of CO2 per annum for every kvar of reactive power.
Which raises a couple of questions.
- Is 100 kilograms a lot?
- What is a kvar and what’s reactive power?
To answer those, let’s look at what happens to power works in electrical circuits.
What happens to power in electrical circuits?
Electrical systems are designed to transfer active power P in watts (or kW or MW) from generators to loads like motors, lamps, computers and heating equipment. That power is then transformed into mechanical power, light or heat. The basic units of electrical system rating are rms voltage (Vrms) and rms current (Irms) The product of Vrms x Irms gives the apparent power S consumed by a load in VA (or kVA or MVA). In an ideal situation, the active power P transferred to the loads should be equal to apparent power S. Unfortunately, this is not the case: P is smaller than S (P<S). In other words, the power factor λ – i.e. a ratio of P/S – is less than 1. The perfect power factor would be 1, but there’s no such thing as perfection. However, a Power Factor that is over, say, 0.95, is good. The reason for low power factors is the circulation of what is called “reactive power“. Mainly associated with motors, its unit of measurement is “var”, “kvar”, or “Mvar”.
How do capacitors improve the power factor?
To bring the power factor to as close to 1 as possible, capacitors are commonly added to the system near the loads. Capacitors act as “reactive power producers”, so that the reactive power does not have to transit along the power line (see diagram). When capacitors bring the power factor up to 1 (or as near as possible), they are said to improve or “correct” or “compensate” the power factor. The result is a fall in the apparent power S circulating along the line and, therefore, in the line current. The circulation of current from the generator to the loads causes line losses, which translates into heat dissipation. These losses are proportional to (Irms)^2 or to S^2 for a given voltage. By reducing the circulation of reactive power through the line, capacitors reduce the line current and associated power losses. Which is how capacitors save energy.
How big are these energy savings?
It is considered that the line losses represent approximately 10% of the total active power transmitted in the electrical systems. A capacitor can save one-third of a line’s power losses. Based on typical system characteristics and average data concerning CO2 emissions per kWh produced, it is possible to estimate the environmental benefit of every kvar of capacitor connected. The result is around 100kg of CO2 saved per year for every kvar.
Is 100kg of CO2 saved per year for every kvar a big deal?
Yes, it is. It is the quantity of CO2 released into the atmosphere when your car burns 40 litres of fuel. One kvar is the common power unit of capacitors, the typical power of capacitor banks connected in the industry being in the range of hundreds or thousands of kvar. So, capacitors prevent hundreds of thousands of tons of CO2 emissions worldwide from being discharged into the air. Of course, capacitors bring other benefits. But that is another story.