SIA "S plus" is building a patented wastewater heat exchanger to reduce the energy costs in multi-apartment residential houses.

Wastewater contains the energy potential, which is obtained by heating it for household needs, for example - when preparing food, washing, etc. It is not appropriate to discharge this potential into sewage systems until it has been fully recovered. The S Plus REC technology has been developed and patented in Latvia and it helps to dissipate and supply the heat energy to the wastewater. By dissipating heat energy from wastewater, it can be used in the building's heat supply - for hot water and heating needs, while the building can be cooled in the summer months by supplying heat energy to the wastewater.

Multi-apartment residential buildings have several constant energy consumption values, which slightly change within the year, it is interesting that these values practically do not change even if the building is renovated, insulated. We are talking about hot water heating and circulating heat. Circulating heat, according to the heat energy consumption calculation methodology, is assumed as a constant value. However, measurements show that often the assumed value differs significantly from the one assumed in the methodology (usually assumed 0.1MWh per apartment, however, in nature these values can vary from 1.5 to 2.5 times). As an example, we will take the apartment house with a total area of 2460m2 and 62 apartments. In Figure No.1 you can see the changes in the building's heat energy, hot water heating and circulation consumption before and after the building insulation.

Figure No.1. Heat demand in the building before and after insulation.

You can see in Figure No.1 that the heating consumption of the building has decreased 2 times, while the amount of heat consumed for hot water preparation and circulation has remained unchanged. This can be partly explained by the fact that the building's hot water risers haven’t been insulated in the project, but you should keep in mind that the bathroom heating (towel dryers), which constantly consumes heat energy, is also connected to the bathroom's hot water system in the apartment building.

The specific building generates on average 10,5m3 of wastewater per day, their temperature at the building outlet varies from 25 to 27*C. By discharging the energy from the building's wastewater, it can be used to prepare the building's hot water and provide circulating heat. Of course, this amount of energy will only be able to partially compensate the heat demand during the heating period. In contrast, in the summer, in addition to dissipating heat from the wastewater, SIA "S plus" technology S Plus REC allows to reheat them with the heat discharged to the indoor cooling building. The energy needed for heating and indoor cooling in a building can be significantly reduced, while the daily use of hot and cold water by the population remains unchanged and is still equivalent to the annual heating demand. In the summer months, in addition to the demand for heating hot water, there is also a demand for indoor cooling. Unfortunately, even in new residential projects, indoor air recuperation and cooling devices are rarely provided, as they create additional operating costs and are only relevant for a few months of the year. The conventional solution is to cool the heated air indoors with "chiller" systems, where the dissipated heat energy is discharged into the atmosphere. The SIA "S plus" alternative solution developed in Latvia provides returning the heat potential of the room to the wastewater heat recovery unit. This means that when heat is removed from the room, it is transferred to the S Plus REC patented wastewater heat recovery tank, where it is discharged through a heat exchanger to increase the wastewater potential used to increase the efficiency of the heat pump for hot water production. A heat pump is a device that simultaneously produces two types of energy: on the A side - heat; on the B side - the cold. Its efficiency is highest when both sides are used. SIA "S plus" has designed a device so that the energy generated by the heat pump at both outlets can be fully utilized by compensating the unevenness of the cold and heat cycles during the day, Figure No.2.

Figure No.2. Daily energy consumption curves in the apartment building, in the summer.

The heat demand for hot water preparation and circulation, as well as the amount of wastewater generated by the building, is a value that varies slightly from year to year. Indoor cooling demand is a value that depends on the outdoor air parameters during the summer period. Under normal conditions, where the building is built according to low energy consumption standards, the hot water and circulating heat capacity will be higher than the recovered wastewater heat and indoor cooling capacity - (Qhot water + Qcirkulation) >= (Qwastewater + Qfreecooling). The purpose of the S Plus REC system is to approximate these values. This means that during the summer months, indoor cooling in the building can be provided by its partial or full compensation with the potential of the cooled wastewater. As the demand for indoor cooling increases, Figure No.2, SIA "S plus" control system will switch the heat pump to "chiller" mode within the period from 15.00 - 18.00, thus the wastewater will not be cooled. As the demand for hot water increases and the indoor cooling capacity decreases, the SIA "S plus" control unit will switch the indoor cooling to free cooling mode, producing hot water from the wastewater tank.

Figure No.3. Sample of system operating parameter settings.

The control unit of the SIA "S plus" wastewater heat recovery system is programmed by combining different devices in one engineering system. Figure No.3 shows the value ranges for which the system operating conditions are set - upper and lower wastewater temperature in the wastewater tank, as well as the optimal temperature corridor at which the control unit switches the heat pump on/off.

Figure No.4. System control display.

In Figure No.4 you can see the control display which shows the instantaneous parameters taking into account the set value ranges.

The efficiency of the S Plus REC wastewater heat exchange system is shown in Figure No.5. The calculation is based on the amount of monthly heat generated by wastewater, daily consumption is planned 10,5 m3/day. The amount of electricity required for a heat pump can be determined by dividing the energy consumption of the building by the efficiency. It is possible to determine the costs by knowing the amount of electricity consumed and the energy tariff - 0,11 EUR/kWh, heat energy tariff - 0,07EUR/kwh.

Figure No.5. The potential of cost reduction for energy supply in apartment buildings.

During the summer, the efficiency of cooling is assumed to be from 21 to 25. This is possible as almost all the heat energy for the building is transferred in free cooling mode, when the only electricity consumer is a circulation pump. When the heat pump is operating in "chiller" mode, its efficiency decreases, but remains high.