Reducing energy consumption of solenoid valves

Keeping the consumption of electricity to a minimum and lowering the environmental ‘footprint’ as it is often called, became more important in the 21^st century. However, when we discuss it, we usually think about the choices of large energy consumers, such as big pumps, electric heaters etc. But, you would be surprised how much energy can be saved, by solenoid valves, especially if you have complicated systems with many solenoids. In this article, we will study the different aspects of solenoid energy consumption and provide guidance in choosing the best solenoid valves for optimized energy consumption.

Standard solenoid valve operation

By applying electricity to a standard solenoid valve, a magnetic field is created in the solenoid. This causes the plunger to be lifted upwards. The stop at the top of the coil blocks the plunger on its upwards movement. But in order to remain open, the coil must receive constant electrical power to maintain the magnetic field, holding the plunger up against the stop.

Factors that affect energy consumption of solenoids

The best time to think about solenoid energy consumption is during the design stage, because many of the factors influencing energy consumption are related to the right choice of valve type and size. Generally, you can assume that solenoids consume energy in one state (either open or closed). The most important factors to consider are:

• Dimension of the solenoid valve: If you over-dimension the valve it might work perfectly, but will consume too much unnecessary energy.
• Design or type of solenoid valve: The difference in energy consumption of different types of solenoid valves can be significant. The following type- and design options influence energy consumption:
  • Normally Open or Normally Closed: This is part of the design of the valve, but deserves special attention as it influences the energy consumption a lot depending on the required cycle pattern.
  • Direct or indirect operated: Usually indirect operated solenoid valves consume less energy but they cannot be used in every application.
• Cycle pattern of the solenoid: The way a solenoid valve is expected to operate, in terms of the number of opening and closing cycles and the times the valves need to stay open or closed, plays a big role in choosing the right model. Each solenoid valve will have completely different energy consumption, based on the cycle pattern. Choosing the right valve for your cycle process will save a lot of energy.
• Peak currents during opening: The current (peak current) to lift the plunger needs to be a lot higher than the current (holding current) necessary to hold the plunger against the stop in the open position.
  • AC or DC power: Depending on the application solenoid valves with AC or DC voltage have different levels of energy consumption.
  • Additional circuitry to reduce the holding current: This can significantly reduce the energy consumption.
  • Latching solenoid valves: A solenoid valve with a built in permanent magnet to avoid holding currents.

Properly dimension a valve based on the system requirements (Kv-value)

It goes without saying that the solenoid valve should be large enough to accommodate the required process flowrates. However, over-dimensioning the solenoid valve will lead to a continuous over-consumption of energy and needs to be avoided. Therefore, the valve size is determined by the flow rate required.

Kv-value

Kv is the flow coefficient, flow rate parameter, or flow factor that is used as a calculation base for the different process conditions. Its unit is m³/h and it is used to define the flow rate of valves.

The Kv-value describes the amount of water (from 5° to 30°C) which flows through a valve with a pressure drop of 1 bar. Use our Kv-value calculator to quickly and easily calculate your Kv-values.

Cv-value

The Cv-value is American indication of Kv. The Cv-value describes the amount of water which flows through a valve with a pressure drop of 1 psi.

Its unit is gallons per minute (gpm) and can be converted to and from Kv by the following conversion formulas:

Formula:

The flow rate (of water) can be calculated when the Kv value and pressure drop over the valve are known:

Where:

• Kv = flow coefficient [m3/h]
• Q = flow rate of the medium [m3/h]
• ρ(rho) = density of the medium [kg/l]
• Δp = pressure drop over the valve [bar]

Design or type of solenoid valve

The difference in energy consumption of different types of solenoid valves can be significant. The following type- and design options influence energy consumption:

‘Normally Open’ or ‘Normally Closed’

‘Normally open’ means that the solenoid valve is open when not actuated. ‘Normally closed’ means it is closed when not actuated. If you choose a ‘normally open’ solenoid valve but due to the cycle pattern the valve has to be closed most of the time, a lot of energy is waisted, because solenoid valves consume energy when actuated. So, if the cycle pattern dictates that the valve needs to be closed the majority of the time, the best choice is a ‘normally closed’ type.

Now, let’s assume your requirements are to have the valve open for half a day and closed the other half of the day and it needs to change its open/closed positions only a few times a day. This means that no matter what type of solenoid valve you choose with respect to ‘Normally Open’ or ‘Normally Closed’, your solenoid would be energized and consuming electricity for half a day. However, a valve that only consumes energy during the opening and closing process (for example an electric ball valve), will consume less energy per day, because when it is in either the open or closed position it does not consume anything. The amount of valve switches is therefore an important factor to take into consideration.

Direct or indirect operated

The force needed to open a direct operated solenoid valve, so the medium can flow from the inlet to the outlet, needs to be provided by the solenoid. With an indirect operated valve, the solenoid only needs to open a very small bleed channel as most of the force to open the main part of the valve is generated from the inlet pressure of the medium itself. Therefore, an indirect operated solenoid valve uses much less energy than a direct operated solenoid valve with the same Kv-value. However, direct operated solenoid valves are less complicated and often a more cost-efficient solution.

Cycle pattern of the solenoid

Your system determines the cycle pattern of the solenoid. The cycle pattern consists of the quantity of openings and closings of your solenoid valve, as well as the amount of time it averagely needs to stay in each position. Different valves have different energy consumption patterns. As you have seen in the previous chapters, design features like NO/NC need to be correctly determined. They need to be compatible with the cycle pattern of the solenoid valve in your system in order to optimize energy consumption.

To determine the energy consumption of specific solenoid valves according to the cycle pattern of your system, you need to do a few simple steps to achieve a comparison table which will help you choose the right solenoid valve. In order to create clarity for yourself on this issue the best way is to make an overview of the daily cycle pattern of each valve:

1) Cycle pattern overview

	Time (hrs.)		Time (hrs.)
Amount of time to open		Amount of time to close
Amount of time in open position		Amount of time in closed position

After you have done that, create an overview of the different valves you are considering, based on their Kv-value that you require:

2) Energy consumption overview

Valve type	Amount of energy consumed to open	Amount of energy consumed to close	Amount of energy consumed in the open position	Amount of energy consumed in the closed position
1
2
Etc.

Combining the results of the two tables will help you determine the right valve with optimized energy consumption for your cycle pattern.

Peak currents during opening

AC or DC power

When choosing between AC and DC power, the following needs to be taken into consideration. The amount of power needed to magnetically raise the plunger to its open position is much larger than the power needed to keep it against the stop in the open position. The power required to hold the plunger in the open position could actually be much lower (20-40% of the opening power).AC-solenoid valves have a natural current peak at opening, whereas DC-solenoid valves have one steady current. As a result, standard DC solenoid valves usually consume more electricity. But there are several ways of reducing holding currents:

Additional circuitry to reduce the holding current

In order to save energy with a DC operated solenoid valve, additional circuitry can be used to generate the peak current needed during actuation. In this case the overall coil can be smaller as it only needs to keep the valve open. The additional circuitry creates a temporary peak current during opening. This helps save consumption although the additional circuitry also consumes a small amount of energy. This circuitry to reduce the opening current, works like a full wave rectifier.

This additional circuitry can be implemented in several ways. Sometimes it is integrated in the coil of the DC-solenoid valve or it can be integrated in a DIN-connector. However, often customers have an existing system when the need for energy saving arises. In this case you can use a separate module that is mounted between de solenoid and the DIN-connector, like a power saver module. Depending on the application up to 40% energy can be saved.

Latching solenoid valves

As already mentioned, a large amount of power is needed to generate the electro-magnetic field to lift the plunger and bring it to its stop, and a small amount of power is needed to keep it there. In a latching solenoid valve the stop at the top of the coil is replaced with a small permanent magnet. The small magnetic field of this permanent magnet is not strong enough to lift the plunger, but once the coil is actuated and the plunger is lifted the permanent magnet is strong enough to hold it in place and the current to the coil can be turned off. This saves a lot of energy. In order to bring the plunger down again a current with reversed polarity needs to be applied to the coil, reversing the magnetic field. Often latching solenoid valves are used in systems for which low energy consumption is crucial, like mobile, battery-powered systems.

Electric actuated ball valves can be an energy-efficient alternative for solenoid valves

While solenoid valves consume energy continuously in one stage, electric actuated ball valves only consume energy during opening and closing movements. During the static position of both stages they consume hardly any energy. Only an insignificant amount of energy is consumed by the actuator relay.

Perhaps an electric ball valve consumes more energy to open or close than a solenoid valve with the same Kv-value (flow coefficient). However, the daily energy consumption of a solenoid valve could be a lot higher than for an electric actuated ball valve, depending on the cycle pattern. If your cycle pattern requires very few switching cycles and long open and or closed periods, an electric actuated ball valve might be a good alternative, for optimized energy consumption.

However, before changing out a solenoid valve for an electric ball valve you need to take the following into consideration. If the power fails, a normal solenoid valve will revert to a default stage (for example: closed). An electrically actuated ball valve however will remain in the previous position. In some applications, this is not desirable and the valve needs to be returned to a safe (for example: closed) position when power fails.