Variable speed drive (VSD) motor control can reduce the energy bill on mechanical services installations of all kinds by more than their own capital cost in a relatively short period - often less than a year. However, the cost of implementing VSDs on electric motors is perceived as being too high for some SMEs. Convincing end-users to spend money on energy saving equipment during an economic slowdown is difficult.
In the UK there is a culture of sweating our manufacturing assets. However, the money saved by not investing in new equipment can soon be offset by rising energy costs. As time passes, inefficient industrial machines consume more energy and begin eating away at profit, argues Steve Brambley, deputy director of Gambica.
"Energy cost will by far outstrip the initial capital cost of machinery. Inefficient machinery reduces profit because 90 per cent of the total lifecycle cost of motors is spent on energy as opposed to 10 per cent on capital cost," reasons Brambley.[Page Break]
"It is often a badge of pride for some manufacturing directors to show to their board how long they have sweated assets for beyond their depreciation period - to eke out the last ounce of production, at times regardless of whether this proves to be profitable or not.
"This applies in particular to heating, ventilation and air conditioning (HVAC) systems, but also to all kinds of markets where pumps and fans are used, whether it is a data centre, a bank, a hospital or a swimming pool. HVAC suppliers can reduce their cost by not having VSD control, but they miss the opportunity to provide an energy benefit to their customers."
Alun James, Industrial VP at Sagentia, a Cambridge-based consulting group listed on the Alternative Investment Market (AIM) in the UK, believes that taking a holistic approach, including advances in sensor technology, provides ultimate smart energy control. Sagentia has developed several technologies, including imaging systems and acoustic signatures, to monitor and control HVAC.
"The key is keeping track of multiple sensing inputs like temperature because there is a lag in the thermal rise of equipment," according to James. "Although commonplace with domestic central heating systems, simple on-off or so-called 'bang-bang' control systems are very inefficient in commercial and industrial environments. There is no intelligence in a bang-bang system: it is purely reactive. Thermal lag causes temperature control systems to oscillate, so there is hysteresis in the system."
Installing many sensors can become quite expensive when simpler measures are just as effective, like using a microphone or accelerometer to indirectly sense the condition of equipment as James explains: "Accelerometers and microphones are almost interchangeable - where a microphone picks up acoustic signals, an accelerometer picks up physical vibrations and the two are so tightly coupled you can use one or the other depending on the nature of the application.
"You can listen to an electric motor's acoustic signature, and you can hear when it is starting to whine or when the load is off-balance. This function can be performed by someone with a trained ear, but alternately machine algorithms can be used to ascertain whether the acoustic signature is right or wrong.
"There is also a lot of information you get from transients and short duration pulses, and in a motor, a transient can be indicative of something instantly failing. Frequency analysis is useful for long-term trend analysis whereas transient analysis can alert a more catastrophic event, like the fact that a rotor shaft has snapped for example.[Page Break]
Two dollar DSP
"Developing an algorithm that extracts all the data and converts it to real information is the key. It is very easy to prove that an algorithm works on a desktop PC, but the challenge is implement this algorithm on a digital signal processor (DSP) costing a couple of dollars."
Ackworth School in Pontefract, West Yorkshire, has reduced its swimming pool energy bills by also taking a holistic approach, using VSDs, remote device monitoring equipment and voltage optimisation/stabilisation. Peter Cheseldine, clerk of works at Ackworth School, recommended to the governors that VSDs be installed to reduce energy costs.
"Getting the financing for VSDs approved by my superiors was an uphill struggle initially," Cheseldine recalls. "However, as energy prices went up, they realised that they should be doing something. Last year, we also went on to install Powermaster's Integrated Water & Energy Management System (IWEMS), which automatically tests swimming pool water quality and dosing."
IWEMS constantly monitors the chemical demands of each individual pool, and recognises when bathing loads have changed. The system signals the VSDs to reduce motor speed when bathing loads are low and chemical demand has fallen, and vice versa when chemical demand rises. Complementing IWEMS is Powermaster's VRS voltage optimiser/stabiliser, which reduces electricity consumption by automatically reducing site voltage to a constant, stable 380V.
"VRS controls the voltage on site to stable levels and gets rid of harmonics, and we are looking to further reduced energy costs in the region of 7-10 per cent," Cheseldine adds. "Return on investment has been quick on this equipment, especially with speed control on the swimming pool pumps, where we are saving anything up to 30 per cent now. Total payback was only about three years, so we have now had five years of pure saving."
Three years payback appears to be typical for VSDs generally, as the case of Associated British Ports (ABP) also shows. Together with its sister port of Grimsby, ABP Immingham is the UK's largest port by tonnage. In order for shipping to operate continuously, the water levels in the port need to remain at a consistently high level, so water must be pumped into the port. The equipment used was first installed in 1973 and was no longer efficient. It controlled the flow of water into the port through the use of three pumps.
The motors could only be operated in on/off mode, with no control of how quickly or slowly the water was pumped in. Furthermore, the equipment was subject to frequent repairs and spare parts were difficult to come by and costly. Siemens calculated the energy savings that ABP could expect by installing a new VSD and motors to allow the speed of water flow into the port to be reduced.[Page Break]
Longer running for better efficiency
Under the old system, running the equipment for seven hours per day, 365 days per year, cost ABP £92,893 per annum, based on a cost of £.0.66 per kWh. By installing the new variable speed equipment and reducing the speed at which it is run down to 70 per cent, but increasing the length of time that it is in operation to ten hours per day, it was predicted that ABP would make a saving of £47,376 per annum.
This meant a predicted saving of more than 50 per cent annually, and a payback time of approximately three years. Taking into account energy cost rises since then, payback time could be even less. Then one day, a motor breakdown on one pump meant that a solution had to be found immediately to minimise the cost of down-time, as the cost of repair would come to a six figures sum.
Since the Siemens equipment was installed an average of 16 maintenance hours per month have been saved. Mark Hennessy, Control Systems Engineer at ABP says: "The system installed by Siemens and Southern and Redfern has made our site more energy efficient, improved operational efficiency and reduced the man hours spent on maintenance."
One of the leading gluttons of energy is of course wherever there are HVAC installations, whether they are in buildings and data centres or in trains, boats and planes. A control system for a hydrofoil in Italy posed a particular challenge for Esposito Orfino, managing director of Syel, an Italian Rockwell Automation partner. He was asked by the manufacturer to optimise electricity consumption and to use only one generator, instead of the two that were usually required to start the electric motor driving the compressor. This could often cause serious voltage drops during the motor starting phase.
"It created black-outs, which our customer regularly experienced, with the possibility of adversely affecting passengers' safety and comfort," Orfino explained. "Another difficult challenge was integrating a new control system into an existing and functioning shipboard's electrical system.
"We suggested that the customer upgrade a star delta starting system with a new 37kW 220V AC Allen-Bradley drive to handle a peak of at least 10 times the rated motor current. In this way, it has been possible to keep only one generator in service instead of the two previously connected in parallel to the shipboard utilities and air conditioning system."[Page Break]
Traditional data centres use computer room air conditioning/handling (CRAC/CRAH) units from various manufacturers with air distributed under floor via vented tiles. Outdoor heat rejection is via dry cooler, condenser or cooling tower typically used in over 90 per cent of installed data centres and still used in over 80 per cent of new data centres.
In certain climates, cooling systems can save over 70 per cent in annual energy costs by operating in 'economizer' mode, corresponding to a reduction of over 15 per cent in annualised power usage effectiveness (PUE), according to Schneider APC. Operating in economizer mode saves energy by utilising outdoor air during colder months of the year allowing refrigerant-based cooling components like chillers and compressors to be shut off or operated at a reduced capacity.
Economizer mode allows refrigerant-based mechanical cooling to serve as the secondary mode of operation or backup. At the DatacenterDynamics London conference in December 2011, Paul-François Cattier, VP for data centre solutions at Schneider APC, said: "The concept of a bypass of the compressor function is central to all economizer modes. Historically, building an economizer mode into a data centre cooling system was only justified in situations with extremely favourable weather conditions, such as high latitudes.
"However, economizer modes are now considered advantageous in almost all locations because data centres spend considerable time operating at light load. An economizer mode must take advantage of a wide range of outdoor conditions in order to maximise the number of economizer mode hours and save energy. However, in periods of extremely hot outdoor conditions, it is necessary to at least partially depend on a refrigerant-based mechanical cooling to reliably maintain data centre environmental conditions while saving energy."
However, the transition between economizer mode and refrigerant-based mode can be very complicated and could result in a temporary loss of cooling during the transition, warned Cattier. Air conditioner bypass via air heat exchanger or via heat wheel economiser modes have the simplest control systems.
The most complex economizer mode control system is the chiller bypass via heat exchanger due to the dead band between the low condenser water temperature required by the plate-and-frame heat exchanger and the higher condenser water temperature required by the chiller.