When Renzo Rosso said “Diesel pioneered the idea of luxury denim, and we still drive this market. But it encompasses more: the consumers love the brand, the lifestyle, the mentality of Diesel”, it certainly had nothing in common with an equally sturdy, vintage, very widely distributed engine technology that was designed to operate originally from a vegetable oil. By this is meant the good old Diesel Engine (not the pants). This remarkable workhorse has been the backbone of the long haul and freight transportation network, as well as supporting the back-up generation system for decades. This longevity will amaze those who believe the late 1800s are prehistoric times, especially if viewed against the waves of disruptive technology of the latter part of the post WWII era.
Diesel and gas generators for back-up power are nowadays easily accessible and come in an extensive variety of designs based upon the application and method of operation. Using diesel as fuel for standby generators costs less than equivalent gas generator models because of cheaper parts and components, including the starting framework, which does not need a spark ignition. Diesel systems are viewed as more rough and dependable in extremely tough operating conditions and used to be prided on having longer working lives.
Those who contemplate the sizing of a generator for business premises normally have to consider the following: the phase (either single or three); the power requirements; and the voltage requirements.
Most business applications require a more prominent surge capacity, particularly when many motor loads are included or various sections perform work independently of each other and turn on and off as often as possible. Large motor inrush current control, typically above 150% of full load, is momentarily a daunting task and the generator has to be fit for purpose to handle this. The generator reserve capacity should be selected so that it can compensate for voltage drops across certain loads in such a way that a 30% voltage drop margin is maintained throughout.
So now that we have established that size matters, what next?
We all strive to be right, but hunches are by no means a replacement for good old empirical science. To do this, take out the meters and do some measurements. Here is a little procedure:
Full load current measurements during peak usage = Itotal
• Use a clamp on meter at the main distribution box, attach 3 phase clamps to all relevant phases and add the currents. Vsupply x (Itotal /3) x 1,000 (kW) for 3 phase
• If the above was single phase then the current component is (Itotal/2)
Use the calculations to estimate the capacity of the genset by measuring total current used by the plant, which, as explained earlier, is all about being in the ballpark for inrush estimation as well.
Clamps on the meters are preferred in all the above, if you value life and limb! From here on a bit of reading is required, and for the sake of brevity the reproduction of the NEC, or equivalent emergency safety allocations for articles 700, 701, 702 and 708, are not reproduced here. If you already know the legally required emergency system requirements for your plant, then just add that in the kW. (If you don’t have it; that could be a serious matter!) This will give you the required safety margins to add to the above calculation to obtain the full load power in kW.
Full Load Kilowatts = Total Amps x Supply Voltage + Emergency Safety Margins
Reserve Capacity = Full Load Kilowatts x 0.25
For 100 percent power, Generator Size = Full Load Kilowatts + Reserve Capacity
In order to compare your maximum power usage make use of your utility bill. Take the peak demand by analysing the data of the year before and add the 25% for reserve capacity. When it comes to shopping malls and the like, floor size matters. The square metre sizing method is an estimation method that is often utilised in retail applications such restaurants. These types of businesses can determine the generator size using the following calculations: Shopping area application: Genset power rating + 100 watts per square metre Other business application: Genset power rating + 50 watts per square metre It is important to note that the actual power requirements of high demand appliances vary widely by brand, model and capacity, and no estimate, as was done above, will capture wide variations if that is expected in a particular application.
Follow the ‘Green Machine’
A range of natural and liquid petroleum gas engines entered the back-up generator scene to cater for the cleaner, greener energy market. Gas gensets are also easily adapted to combined heat and power applications to boost for even higher energy efficiency by incorporating heat exchangers or chillers into the design.
Most of these gensets indicated above provide high horsepower and low emissions. Gas gensets are based on proven technology and manufacturers strive to pack robustness, reliability and lower energy wastage into these compact units. Some even take pride in providing the latest solid-state ignition systems and embedded micro-electronic engine supervisory systems.
Where gas gensets can give one a headache is in the gas supply considerations that are required. Gas piping needs to be properly sized to enable the delivery of the engine’s required gas volume. This must also be delivered at the correct pressure to the generator. A careful study of the correct gas supply pressure should be made and this must be installed according the manufacturer’s requirements. Accurate and adequately sized gas meters should also be installed on these lines. All piping must be sized to carry the correct volume of gas flow to support the generator at full load.
A deficiency in any of the above requirements will lead to poor engine performance and even starting problems. This then leads to limited generator output and significant lag when adding higher loading. The engine’s lifespan will shorten drastically, because it will now be a ‘lean-burning’ engine, which in turn causes the engine to run at higher temperatures than normal. Welcome to the world of premature engine failure, if you thought ‘lean burning’ was a snazzy ‘green energy’ term; it’s not! And the repair bill will make sure you never forget.
Commercial back-up generators keep businesses operating even after the utility supplied power goes out. Standby generators for commercial applications keep doors open for business to serve customers during an outage that can last hours, days or even weeks. Reputable brand names and back-up power equipment meet the most demanding needs of virtually any commercial operation including stores, offices, restaurants, and small to medium manufacturing and industrial operations. When the power goes out, standby generators automatically start within as little as ten seconds. Employees keep working and business continues as usual. ESI.
ABOUT THE AUTHOR
Prof Mohamed Tariq Kahn is the head of the Centre for Distributed Power and Electronic Systems at Cape Peninsula University of Technology. The Centre researches alternative energy solutions, microgrid and smart grid deployment.