Being Green: VFD vs CSV
Making the electric meter spin slower is not the only way to save energy. Sometimes what we do to make our electric meter spin slower uses more energy somewhere else. Every action has an equal and opposite reaction.
Raw materials for manufacturing products cost very little. Most of the cost of any product is for the energy used to mine, manufacture, transport, install, and recycle. About 90 cents of every dollar spent on consumer products is for the energy used to make the product. This can be verified by the fact that when the American public stopped excessive spending on consumer products, energy consumption decreased dramatically, and energy prices fell proportionally. Trying to spend our way back into prosperity will only make big corporation more prosperous. Reducing the amount of consumer products purchased is the best thing a consumer can do to save energy, and reduce our carbon footprint.
The Cycle Stop Valve or CSV can save money and energy directly and indirectly. A Variable Frequency Drive or VFD system may use a little less energy than a CSV system at low flow, but the parasitic losses and reduced efficiency of the motor running on a VFD, means the VFD uses more energy at high flow. Over the full range of flow, the amp draw or energy reduction that happens as a CSV throttles the output of a pump can be almost exactly comparable to the energy reduction of a VFD system. To verify this you only need to use the standard Brake Horse Power equation. If you know your flow rate, head, and efficiency the horsepower can be easily figured using our Horse Power Calculator.
However, there is nothing more efficient that a properly sized pump running at it's best efficiency point. Any time you reduce the RPM with a VFD, or throttle the output with a valve, a pump is using more energy per gallon produced, than when the pump is running at it's best efficiency point and full RPM. There are cases where a CSV or VFD can save energy directly. When the flow rate required is between 60% to 90% of the max flow, and rapid cycling into a pressure tank causes multiple motor starts with high inrush currents, a CSV or VFD will reduce the energy consumption directly. The lower the flow rate required, the more energy per gallon is used with a VFD or a CSV. Because head is lost by the square of the pump speed, when a static head or constant pressure must be maintained, a pump cannot be slowed down enough with a VFD, to save anymore energy than a CSV.
To see the indirect ways that a CSV saves energy, we must compare it to the other types of pump control. Cycling on and off into a pressure tank is one of the fastest ways to destroy a pump system. Because of cycling, the average life of small submersible pumps is only about 7 years. Those that cycle the most only last 2 years, while those that cycle the least last about 12 years, hence a 7 year average. Even with a fairly large pressure tank, cycling destroys every component in a pump system. Cycling flexes the bladder in a pressure tank, until the bladder breaks like bending a wire back and forth. Cycling destroys pressure switches, start capacitors and starting relays. Cycling torques the pipe in the well, until the down hole wire is rubbed bare. Cycling can strip pump splines and loosen impellers. Cycling also destroys the motor or strips the motor splines.
VFD's are another reason for premature pump system failures. Although they have been around since 1968, VFD's have recently been pushed into the domestic pump market. With false claims of energy savings and extended motor life, many people have fallen for a VFD system. Reducing the RPM of a pump causes more energy use per gallon, not less. Claims of a soft start from a VFD extending motor life are greatly exaggerated. It is true that the VFD reduces inrush currents, which can technically allow a motor to survive multiple starts. However, the pulsing DC voltage of the VFD, and the rapid rate of voltage rise, creates high voltage spikes to the motor, which are amplified even more as the length of motor wires increases. Harmonic currents produced by the VFD also create additional heat in the motor windings. Increases in the turn to turn voltage in the motor causes partial discharges. Partial discharges, burn off winding insulation and hastens the demise of the motor. Increased heat caused by the harmonic currents requires additional cooling, and further shortens the life of the motor.
The VFD itself generates heat and requires cooling. Lint, dust, bugs, or a fan failure, can be just as destructive to the VFD as lightning and voltage problems. The fact that a VFD system does not utilize any draw down from a pressure tank, means the pump must start every time a tap is cracked open. Although it is a soft start, it still torques the pump on start up. These multiple torques on start up can quickly rub bare spots in the drop wire, and can strip motor splines. Multiple starts also quickly destroy check valves. Varying the speed, also runs the pump and motor through resonance frequencies that causes destructive vibration. Any of these things will cause the pump system to fail prematurely. We should also consider the problems caused by the use of Coltan and other metals. Some of which are mined and smuggled from third world countries to make electronics like VFD's. In the 10 years that VFD's have been pushed on the domestic market, many manufacturers are working on their 5th or 6th generation, trying to solve some of these problems. While there may be a few VFD's in lightly used systems that have lasted 10 years, most average about 3 years before a replacement is needed.
Since cycling is one cause of most pump system failures, eliminating 80% of the cycles with a Cycle Stop Valve, can triple or quadruple the life of every component in the pump system, compared to a pressure tank only system. Because a CSV system runs on standard sinusoidal power at a constant RPM, there are no voltage spikes, harmonics, resonance frequencies, or additional heat produced. A CSV system can more than quadruple the life of a pump system when compared to a VFD. A pump system that last 15 years, can save a tremendous amount of energy over having to replace the pump system every 5 years. Taking into consideration that the CSV system uses a pressure tank that is a fraction of the size and cost of larger pressure tanks, can add substantially to the savings. Reductions in square footage needed to house a large pressure tank, and saving the heat required for that extra square footage, can add even more to the energy savings. Lastly consider the energy used to mine, manufacture, transport, install, and recycle the additional pump systems destroyed by cycling or VFD's.
Our energy footprint is bigger than we think. Using a CSV to make pump systems last longer, can save more energy than just making the electric meter spin a little slower. Being green is all about saving energy, and reducing the amount of consumer products used, is the best way to save energy.