Warsaw Business Center

The Application Context

The Warsaw Business Centre, located in the historic centre of Warsaw, Poland, is based on the renovation of a mid-twentieth century building previously used as a state administrative office. Composed of several blocks, it is now the headquarters of start-ups and detachments of international companies.

Customer Requests

The building, built between the 1940s and 1950s, has recently undergone a modernisation of its systems, including the ventilation system. The need was to be able to manage each wing of the building independently, monitoring the levels of air quality and preventing peaks in electricity consumption due to the simultaneous operation of ventilation in multiple areas.

Ekinex Solutions

The control and monitoring of the ventilation system and the energy loads was entrusted to the Ekinex EK-IA1-TP PLC. Each area of the building has been equipped with an inverter-controlled extractor/exhaust fan and a CO2 detector that measures air quality; if the CO2 concentration exceeds a threshold value of S1, the ventilation system must be activated. In order to prevent as much as possible the peak of electricity consumption due to the simultaneous operation of the ventilation of several zones, it was decided - if the overall consumption is less than a certain level of power - to anticipate the intervention of the fan to a pre-threshold S2 (for which the air quality is still considered adequate). In this way, the condition in which the intervention of the ventilation is indispensable is avoided, exploiting the periods of lower consumption to turn down the level of CO2 in advance. In addition, if the "pre-ventilation" request is not met within a period of 30 minutes (or the CO2 concentration reaches the upper limit threshold), the ventilation is still started without limiting consumption.

In practice, the activation scheme can be summarised as these rules (called C the CO2 concentration):

• if C is lower than S2, the fan is switched off;
• if C exceeds S2, and the total power is lower than Pmax, the ventilation is started;
• if C is kept above S2, but the total power continues to be higher than Pmax, after 30 minutes the ventilation is still started;
• in any case, regardless of consumption, if C exceeds S1 the ventilation is started.

For the calculation of the overall consumption, each fan is associated with a corresponding value of electrical power, which for simplicity we consider fixed; in reality, this value could be measured by a load analyser. To add a degree of complexity to the mechanism, let's suppose that each of the zones is equipped with an internal temperature sensor , and that we have an external temperature sensor at our disposal: we would like to make sure that, if the internal/external temperature differential were too high (e.g. greater than 20°C), the fan would run at a reduced speed, to avoid overloading the heating (or cooling) systems. Below is a simplified explanation of the implementation of the system for the sole management of ventilation on the basis of the CO2 levels detected and the control of loads.

In reality, the system has considered other factors, such as the regulatory factors that envisage adapting the ventilation to the number of people present in the building and other variables. Let's start by formulating the calculation of the power used by the fans currently in operation (let's assume 3 zones so as not to weigh down the figures):

Let's now consider a sample zone (in the example zone n.1). Let's check the first criterion, that is if the concentration in the zone exceeds the preventive threshold:

If the intervention of the fan is required, we calculate the expected consumption also activating the ventilation of that area:

Note how, in the previous calculation, it should be considered if the zone is already active. In this case, the total consumption already includes the consumption of the area itself, so the "expected" consumption is already the current one and the consumption of the area should not be added a second time.

Let us therefore complete the verification of all the criteria, that is to say:

• if the consumption, by activating the ventilation of the area, does not exceed the desired level;
• if there is not a request pending for more than 30 minutes;
• above all, if the maximum tolerable threshold of CO2 is not exceeded.
  
  

The occurrence of any of the above conditions forces the fan to start up:

Let us now examine the further conditions of the plant. First, we calculate the internal/external temperature differential and check that it is not higher than the selected limit (in which case we will limit the speed of the fan):

Note that the mechanism must work in the same way in the case of outside air that is cooler and warmer. Therefore, let's generate the fan control reference according to the previous condition: The implementation of our model is so complete. Obviously, while remaining within the scope of simplification, we could have made the logic even more complete (for example, considering that, when a fan runs at low speed, the value of its expected consumption must be different, or modulating the operating speed in proportion to the temperature difference). These refinements are certainly possible with relative ease, but they would have been superfluous compared to the main purpose of this discussion, which is to demonstrate how the use of the PLC allows you to easily implement even apparently complex or articulated mechanisms, very difficult to achieve with the only functions integrated in individual devices.