SIA "S plus" proposal for the energy supply of multi-apartment residential houses in the ECO-Trigeneration mode.

An apartment building is a large energy consumer, but it can also serve as an energy producer. In order to reduce a proportion of the fossil fuels in the energy balance for the buildings, SIA "S plus" offers a concept in which heat for the heating and hot water needs in the building is provided by means of a wastewater the heat recovery system. Indoor cooling is provided from a wastewater in free cooling mode. Electricity for these processes has been obtained from the sun, this concept can be called ECO-trigeneration.

The heat pump on the A side produces heat (for the heating, hot water, production process, etc.), on the B side it generates cold (for the indoor cooling, production process, etc.). Figure 1 shows the basic scheme of the equipment connection. The figure shows that the wastewater generated in the building enters the wastewater heat recovery device developed by SIA "S plus", from which the heat is passed to the heat pump on the B side, which is used for building heating and hot water through the A side. When the heat pump is switched on at the same time as the building's cooling system, the cold part from the B side enters the building's cooling system directly, by providing indoor cooling.

Figure 1. Conceptual diagram of the ECO-Trigeneration connection system.

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Figure 2. analyzes the wastewater consumption of the existing building (which is converted into a unit of recoverable heat), the expected cooling demand of the building, the demand for hot water and the benefit from the electrical power generated by 30 m2 solar panels. Figure 2. shows the interaction of the building energy demand in a month of July. Solar radiation in the summer covers a large part of the building's electricity demand. This means that regrouping resource use habits throughout the day, building can be provided a with low-cost renewable energy.

Figure 2. Estimated energy consumption in an apartment building, in July.

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A wastewater heat recovery device can provide the heat energy for a heat pump for a hot water preparation in the morning from the wastewater tank. Solar radiation is sufficient from 4.00 - 9.00 am to provide hot water heat demand for heat pump and heat energy circulation pumps. If the wastewater potential in the wastewater tank from the wastewater generated in the building is insufficient, it is possible to supply wastewater from the city network to compensate it for the lack of energy. From 9.00 - 12.00 am when the residents of the house go to work, the consumption of wastewater decreases (thus also the need for hot water), however, at this moment the demand for indoor cooling increases. Indoor cooling is initially provided by the B side of the heat pump, to the extent that it covers the hot water demand. As the solar activity increases, the cooling capacity increases, and the heat produced by the heat pump on the A side increases accordingly. It is not economically justified to place a large heat accumulators in the building, therefore in the period from 12.00 - 16.00 pm indoor cooling must be provided in free cooling mode, including from the city sewage network, because the demand for hot water in the building is low, so there is no need to operate a heat pump, thus wastefully accumulating heat energy without economic justification. From 16.00 - 20.00 pm the demand for hot water in the building increases, respectively the heat pump works to prepare hot water by discharging the A side in into a heat accumulator and the B side provides indoor cooling (in case of lack of heat and cooling capacity, wastewater is supplied from city networks for energy compensation). In this mode of operation, the wastewater is not cooled in the storage tank by allowing the wastewater heat resource to be used at the end of the day from 20.00 to 24.00 pm after solar radiation, when it serves to prepare hot water in the evening and morning period. This is a way to produce the energy needed for a building during the summer period of the ECO Trigeneration cycle.

The energy balance of the building in the winter month is shown in Figure 3. The figure shows that it is not enough to provide heating of the building with locally generated wastewater during the winter months, solar radiation is also low. Wastewater heat recovery to meet the heating season's heat demand is only applicable in very low energy buildings, where there is also a demand for indoor cooling, as well as in conditions where the consumer is surrounded by several wastewater generating facilities. Even in the winter months, it is useful to use all available renewable energy sources to replace fossil fuels. SIA "S plus" heat recovery system controls the amount and temperature of heat discharged into the wastewater, preventing low-temperature wastewater from entering into the cities sewage network system. On the other hand, it is pointless in our mind to discharge heat resources into networks, which can be reused in energy production.

Attēls Nr.3. Paredzamais enerģijas patēriņš daudzdzīvokļu  ēkā, janvāra mēnesī.

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Figure 3. shows that in the winter months in Latvian climatic conditions, in the period, from 9.00 am to 16.00 pm it is possible to use solar energy, which facilitates the operation of the heat pump and circulation pump with electricity. During the heating season, it is not economically justified to produce hot water in the building during this cycle; the heat produced by the heat pump should be diverted only to meet the heating demand.

Figure 4. Estimated energy consumption in the building.

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In Figure 4. the estimated annual energy consumption of the building is reflected. The figure does not include the solar activity energy curve, but it does reflect the amount of energy produced and demanded in the building. With the ECO Trigeneration assembly complex, it would be possible to save energy costs and significantly reduce CO2 emissions every year. Along with the requirements of CO2 reduction, in the future it will also be necessary to reduce the comfort requirements of human indoor microclimate, with the SIA "S plus" ECO Trigeneration system, it will be possible to maintain comfort requirements at a reasonable price.