Payback periods for energy-saving equipment.
The practice of using energy-saving equipment, namely frequency converters, which are used to change the performance of fans, pumps, compressors, shows that this type of equipment often belongs to the «extra» category, since air or water can be supplied without these devices, and in the case of a requirement for flow regulation, traditional methods can be used.
In the overwhelming majority of cases, customers are initially scared by the high cost of the same ventilation unit with the possibility of frequency regulation compared to, for example, traditional air flow control using a damper. At the same time, the number of system elements is not taken into account at all, the need for which will often disappear when installing a frequency regulator, and therefore, the increase in the cost of the entire system does not seem so significant.
Despite the fact that flow control using converters is one of the most expensive methods in terms of initial investment, but as a result, choosing it will give undeniable advantages not only in the future due to energy savings, but also in the present due to improved comfort indicators in the room. Maintaining microclimate parameters using this method is the most accurate and optimal of all possible, and in addition, the noise level of ventilation equipment is reduced.
Returning to energy savings, the basic principle of using frequency regulation is as follows: the consumed power is proportional to the flow in a cube, i.e. we are dealing with a cubic dependence. In practice, this means that with a typical flow rate of 80%, the power consumption with the traditional regulation method will be at a level of about 90%, while with the frequency method it will be only 51%, and with a decrease in flow rate from 80% to 50%, the power consumption with frequency regulation will decrease to 12.5%.
In real systems, depending on various parameters, namely: time of day, time of year, ambient temperature outside the premises, number of people in the building and other changing conditions, it is necessary to provide the required air supply or the required pressure for pumping systems. Therefore, each such system has a load cycle. In some cases they are typical, in others they are strictly individual, and the data for calculation can only be taken during operation.
VLT® Energy Box software product, developed by the Danish company Danfoss A/S, will allow you to calculate the effect of energy savings and calculate the payback period of a system with a frequency converter based on simple calculations, the algorithm of which is embedded in this software product. All you need to do is go through a few simple steps and fill in the data about your system.
The program allows you to operate both pump systems and fan systems. In the first window, you are asked to specify the design pressure, set the fan characteristics, its load cycle and operating time, and enter basic information about the equipment used: electric motor parameters, frequency converter parameters, its cost, and one of the key parameters — the cost of electricity per kW•h.
In our example, we will consider the implementation of a system with Danfoss VLT® frequency converters on cooling tower fans. In addition to the data presented in the picture, we also take into account that when using a converter, there will be no need for a star-delta start, since our device will perform a soft start, with a much lower mechanical load on the mechanism and lower starting currents, in addition, in other cases with pumping applications, we can also get rid of the flow meter, overload relay and external controller required to control this system. The frequency converter takes on all these functions. In our case, the comparison is with a traditional system with a constant flow.
From the calculation we see that for a small 2.2 kW fan the real costs of a frequency drive system are actually significantly less than the cost of the converter itself, sometimes its cost can simply be washed out in comparison with the effects that are achieved thanks to it, for example, increased reliability, reduced number of components, etc.
The payback period in our example was less than one and a half years. In the West, due to more expensive electricity, energy-saving equipment would pay for itself even faster. Taking into account the long service life of the building and the increase in electricity prices, it is possible to assume, and with the help of the VLT® Energy Box program even calculate exactly how much the operating costs of buildings can be reduced by implementing this type of equipment in the future. Our example shows savings for the next 5 years.
We have considered the energy saving effect only on the example of one unit of low-power equipment, but frequency converters can be installed on most engineering mechanical systems in a building, and in addition, be part of a dispatching system with the ability to communicate via Lonworks, Bacnet, etc. Using the VLT® Energy Box, you can evaluate the comprehensive energy saving effect and consistently calculate all elements of the engineering system and obtain real amounts of initial investment and reduced operating costs in the future.
