Huge amounts of energy can be saved in in the HVAC industry through the use of variable speed drives. A mere 20% drop in speed of centrifugal pumps and fans can save as much as 50% in energy. Yet, many buildings still use traditional mechanical devices to control system flow and therefore have considerable potential for saving energy.
Many heating, cooling and ventilation distribution systems operate at a constant flow rate, even though peak demand may only be required for a few hours. The conventional response to meeting the changing demand for heating and cooling within a building is to restrict flow to individual rooms, while maintaining peak flow in the central HVAC system. However, through the use of this approach, considerable energy is used and equipment lifetime is shortened.
A much better approach is to use a variable speed drive on HVAC pumps and fans to vary air or water flow to meet more precisely changing load demands.
Savings in running costs
Pumps and fans offer the best energy savings potential of any equipment in the building services industry. A fan running at 80% speed only uses 50% of the energy, compared to one running at full speed. Yet far too many pumps and fans run continuously at full speed, with the output regulated by inefficient throttling devices such as vanes or valves. A mere 20% reduction in fan speed can reduce energy consumption by up to 50%. Applied to a 75 kW motor in continuous duty, this means nearly £15,000 per year in saved energy - on one single application.
It is estimated that £1,000 million is spent each year on running electrical motors in commercial applications, excluding industry, in the UK. As much as 20% of this could be wasted through the use of inefficient throttling mechanisms.
Energy saving opportunities with variable speed drives
Variable speed drives are designed for optimum energy efficiency. Without a variable speed drive, the a.c. motor runs at full speed all the time. Variable speed drives enable the speed of pump and fan motors to be infinitely variable Drives operate by switching the fixed mains supply voltage to a variable voltage and frequency f in response to an electrical control signal. When coupled to a fan or pump motor, the change in frequency will result in a corresponding change in motor speed.
Variable speed drives are most commonly used on supply and extract fans for variable air volume systems, circulating pumps in hydronic systems and water booster-pumps in high-rise buildings.
In most cases, the motors are controlled to maintain a constant pressure within air ducts or water pipes. A pressure sensor in the pipe or duct measures the system pressure and as this changes sends a signal to the building automation system, which in turn sends a speed demand signal to the drive. Thus, as valves and dampers close, the pressure rises in the ducts or pipes, this in turn reduces the speed of the fan or motor.
Retrofitting
There are enormous opportunities for retrofitting drives in variable flow applications across the HVAC industry. It has been calculated that only one in four motors used in HVAC applications are controlled by an AC drive. This means that there are many pumps and fans that could benefit from being controlled by a variable speed drive.
Although the initial capital investment is often higher, the amount of energy saved by substituting inefficient control methods such as vanes and valves can result in large energy savings and short payback periods. Additional benefits include higher comfort levels for staff because of better temperature and ventilation control, along with lower noise levels and reduced maintenance costs
As an example, retrofitting boiler forced draft fans with variable speed drives could provide two distinct benefits:
Firstly, the use of a variable speed drive would provide a significant reduction in fan output, particularly when the fan is operated at partial loads to accommodate boiler turn down.
Secondly, boiler fans with variable speed drives and sophisticated microprocessor based control systems can deliver simpler control strategies and improved performance.
Estimating running costs
Pumps and fans are the best applications for variable speed drive retrofits. The best way of determining the cost effectiveness of a potential variable speed drive retrofit is to look at the power needed at each operating condition firstly with and then without a variable speed drive.
Proposed energy savings can then be calculated by taking the reduction in power at each condition and estimating the savings based on the actual or expected operating time of that condition.
Examples of applications for variable speed control are those which: -
- Have a single large pump or fan rather than a series of staged pumps or fans that come on sequentially as the process needs increase
- Have variable flow, where throttling (by valves or dampers) provides the variation and where the majority of the operation is below the design flow.
- Where the operating hours are more than 8 hours per day
Energy audit
In order for a company to reduce energy costs, it needs to evaluate how it uses energy and in what way it can make its operations more efficient. Before you can make the savings you want, you need to establish just what your current energy usage actually is. This is usually done in the form of an energy audit.
An energy audit is a systematic examination of key pump and fan applications that include the monitoring of energy consumed both before and after the change to variable speed drives.
It defines where energy can be saved and quantifies how much energy can be saved with the installation of variable speed drives. These figures are then translated into a possible monthly saving, the amount of money that will be saved, in energy bills alone, if the equipment is installed.
Replacing existing drives improves efficiency
Existing drives should also be considered for replacement, even if they have not actually failed. An old drive could be costing money unnecessarily, compared to more modern and efficient products.
Today’s drives are at least 3% more efficient than their predecessors. For example, a new 75 kW AC drive from ABB’s new range would consume 6,704,082 kWh of energy, at a cost of £301,684, over a 10 year period. A 75 kW drive from the older SAMI STAR range would consume 6,915,789 kWh, at a cost of £311,211, over the same period. This is a saving of almost £1,000 per year.
Replacing old drives brings several benefits to the HVAC user:
- Increased energy efficiency
- Reduced running costs
- Frees up floor space
Variable speed drives not only decrease energy use as well as lessen the burden on the environment, they also qualify for the Government’s Enhanced Capital Allowance scheme. This grants 100% capital write-off in the first year.
It is not unusual for user to dismiss the promise of 50% energy saving on a 20% speed reduction as the exaggerated claims of a manufacturer. However the savings can be verified and the best way to start is with an energy survey. This will enable you to see the potential savings in black and white, enabling you to make the decisions that bring your company improved profitability.