The Best Western hotel in Arjang, Sweden, can be considered as one of the buildings that has opened the way to the “Demand Controlled Ventilation” (DCV) technology.
Ideally, the hotels are designed to work 24/7, i.e. 24 hours for 7 days in a week. However the demand of the highest indoor air quality (IAQ), such as temperature, relative humidity, and CO2, arises when the people are in the hotel rooms, and that is mostly during evenings (late afternoons) and nights. So, if the rooms are ventilated 24 hours per day, independently if they are occupied or not, the energy consumption increase as well as the running cost. It is easy to think how the running costs could be reduced drastically if one can supply fresh and conditioned air only when it is strictly necessary, that is when people are in the rooms.
Besides, if one can consider that the hotel rooms are not occupied all the year, it is a good practice to use the DCV systems which have of course the aim to supply the exact air flow demand based on the immediate request; paying attention to the indoor quality which is one of the most important characteristics for the hotel success.
Anyway, a hotel hasn’t got the hotel rooms only, it has common spaces and in these areas the number of people varied constantly, so the constant air ventilation (CAV) systems are used the most. This usually includes the possibility to increase (boost) the air flow by a timer as well. The air quality reached in these common spaces and the low energy consumption usually does not justify the higher installation costs of the DCV systems if compared to the use and installation of the CAV systems.
Description of a building
The case of study is a hotel building with 6 floors: in basement there are all the technical rooms for the air handling units (AHU), for the district heating and a chiller, also there are laundry room and the staff’s dressing rooms. Ground floor contains a reception, restaurant, and kitchen and bowling space. The first floor includes the offices and guest rooms. From the third to sixth floors there are offices, hotel rooms and conference rooms.
The building is made of a concrete and steel structure made with insulation and fire-resistant materials. The insulation is suitable to reduce the thermal transmission losses through the opaque walls with a thermal conductivity of 0.195 W/m2K, while the glass parts have not as good quality compared to the other parts of the building (the thermal conductivity of1.3 W/m2K) and the guests complains about a cold draught in winter and hot in summer. One can also thing that the shading factor (g-value) is also not suitable to the needs. The shading in rooms is made by internal awnings.
The electrical power is supplied by the net, while the heating energy for the air conditioning as well as the domestic hot water is supplied by district heating. For the cooling power, a chiller is installed with nominal power of 207 kW.
Ventilation and air-conditioning systems
The air conditioning system is 4 ducting type. It has to provide heating or cooling depending on the requirements of the building. For example: during the middle season the hotel rooms exposed to north could require for less heating to reach the set points, while hotel rooms exposed to the south need more cooling according to the best comfort asked by a guest.
The building can be split in 2 zones, each of them is provided by one air handling unit (AHU 1 and 2) equipped with high efficiency rotary heat exchangers. The whole hotel, the kitchen excluded, represents the Zone 1.
In this zone there are three different types of rooms where the heating and ventilation solutions are different too (supplied by AHU 1). Zone 1a has small hotel rooms where the solution is installed with a comfort module, induction active air diffusers for heating and cooling of the rooms. Zone 1b has large rooms equipped with radiators and active climate beams to heat and cool while air mixing diffusers supply cold air. The conference rooms, suites and bowling belong to this room category. And Zone 2c contains common places such as a restaurant and hall/lobby belonging to this category. The comfort is completely assured by the additional thermal load given by the parietal radiators located at the glass base of windows.
The Zone 1b and Zone 1c categories are characterized by the CAV technology with temporized boost for the overcrowding situation. This system is defined as variable air ventilation (VAV) system. The Zone 1a category is characterized by the DCV technology with air-water control modulation.
The Zone 2 consists of a kitchen. The extracted air from the kitchen has to be distinguished in the one coming from the ovens cooking areas and the others zones. The air coming from the cooking zones is not sent to the heat recovery system but, as it is overfilled of greases and smells, it is directly expelled out to avoid damages to the heat recovery system such as reducing the efficiency and the heat recovery capacity and to avoid recirculation of smells in the building. The air coming from the common kitchen area and from the dishwasher hood is sent to the recovery wheel (supplied by a small size of AHU 2) in order to recover its energy.
Hotel rooms and DCV system
The access to the hotel rooms is allowed by an electronic key which has to be inserted in a specific slot once the room is occupied, in this way this solution can be used also as an occupancy sensor in a room: when a key is inserted in a slot this means that a room is occupied, otherwise is empty.
In addition to the heating air system a floor heating in the bathroom was installed to secure additional comfort. At first, the floor heating was switched on when the key card was in the slot, however costumers complained about cold floor in the bathroom, so this control was cancelled in order to function 24/7, which is not very energy friendly.
The indoor room temperature is prefixed, but the guest can modify this parameter according to his/her own requirements.
In the sub-ceiling is installed the air adduction system involving a four pipes air-water heat exchanger in order to let in the room air flows based on hot or cold water according to the need. This component allows the mixing of supply air, incoming directly from the AHU, and the recirculating air. Then this air flow passes through the coil to modify/increase/decrease the temperature to the desired inlet air temperature.
Looking to the previous image (A) is a primary air flow, (B) is a recirculation air at the room conditions, (C) is inlet air flow, as mix of primary and recirculation air, normally composed of a quantity of ¼ from primary air and ¾ from recirculating air.
A dumper on the supply airflow, controlled by the room sensor, sets the amount of fresh air supplied in the room. This dumper is strictly linked with the valves of the heat exchanger in order to modulate the inlet air temperature to the required conditions with a modulation of both flux of air and water due respectively to the percentage of the opening of damper and valves. The system has in memory a set value for opening of these two fundamental components.
Let’s think about the conditions of an occupied room with equilibrium between indoor climate parameters and set point values. Imagine that the room now gets empty. The control system recognizes this because of the sensor on the electronic key.
Because of this there is no need any more for supply of fresh air, the system reduces the supply air flow rate, however one cannot reach the zero air flow because this is a fan free system and so one needs a tiny flux to hold the induction phenomena active. This is necessary to keep the room heated, moreover, in order to save energy, a 2 K temperature decrease/increase (according to the season) is allowed before the heating/cooling system starts to send energy to the room.
The system is now in equilibrium in the conditions of an empty room. Now a guest enters in a room and inserts his key card in the occupancy sensor. The system answers by supplying additional power in boost mode for a prefixed time in order to reach as soon as possible the set point for full room conditions. After that the control system modulates the dumper and valves to the prefixed values.
Another aspect which characterizes this system is an induction phenomenon. It sends a high quality mixing for the supply and recirculating air, moreover the absence of fans ensures an absolute quietness, which is one more point for the indoor comfort score. In all these movements, both supply and extraction dumpers (marked with D on a sketch), placed in the bathroom, adapt coherently their opening to hold the room constantly at the design temperature and pressure.
This regulation system is valid for all the hotel rooms. This is in connection with a central system which acquires information from the rooms and modulates the airflow rate from the AHU adapting the air flow rate with the real need.
Monitoring of the building
The building was built 9 years ago and there is no direct AHU monitoring system. However control equipment is installed in a case of fault- and mal-functioning. It is connected to the AHU and to the other incoming energy sources; electrical, district heating, cooling and ACS. Shortly, the monitoring equipment will be improved to allow a detailed analysis of the building. However, monitoring systems are installed in the control system which links the AHU to the rooms which provides, continuously, data for air pressure, air temperature and air flow.
To monitor the indoor climate conditions three hotel rooms have been chosen at the middle of the corridor on the third floor because of the boundary conditions being adiabatic in all the walls except the external wall. In these three rooms, temperature sensors on the primary air, on the recirculated air and on the incoming air have been installed. Moreover, the CO2, the DHW consumption, the electrical power and the people in the room are continuously monitored. A PC located in one of the rooms is reachable by internet in order to control all the data and download the records.
These records give important data about the hotel guest habits in order to follow up on the air systems and IAQ in more and more fitted on the man profile and in order to avoid waste of energy, trying to invert the trend that characterize the consumerism age.