## 4 Active Systems  
### Active Systems for Indoor Space Conditioning  
The vicinity of the Stadium with the seacoast and the significant indoor space heating and cooling needs create the prerequisites for the installation of an open loop geothermal system, which will operate with seawater, pumped from and disposed back to the aquifer. The geothermal system will co-operate with water-to-water GHPs. The basic structure of the proposed geothermal system is depicted Figure 9[7; 55].  
According to the heating and cooling loads' calculation results after the existing openings replacement, the annual peak thermal power demand equals to 868 kW and 680 kW for heating and cooling, respectively. A commercial GHP model is selected with final heating and cooling nominal capacity of 110 kW and 90 kW, respectively. In total, eight (8) GHP units should be installed in two similar, parallel groups, as presented in Figure 9.  
According to Figure 9, through a closed, primary loop, seawater is provided for the source heat tank, pumped from the aquifer by two independent supply boreholes and cleaned in two hydro-cyclone devices, where it is separated from algae, sand, stones and, generally, any type of debris. Once heat has been provided for or absorbed from the secondary heat pumps loop, during winter and summer, respectively, seawater is disposed back to the aquifer through the disposal boreholes. The heated or cooled water is then pumped, through the secondary closed loop, from the source heat tank to the GHPs' condenser, to offer or remove heat, during winter or summer respectively. The produced heating or cooling in the GHPs is then provided for the load heat tank, through a third closed loop, from which it is finally provided for the thermal energy distribution network. The supply and disposal boreholes should be at a minimum distance of roughly 50 m, to eliminate any possible impact on the supplied seawater temperature from the disposed seawater [11].  
The electricity consumption for the indoor space heating and cooling with the introduction of the proposed geothermal system and the implementation of the proposed passive measure (openings replacement) is calculated using relationship (4) and following the data and parameters below:  
* the COP and EER fluctuation curves versus the seawater temperature of the employed GHP model, presented in Figure 10;
* the heating and cooling loads annual time series after the replacement of selected openings (Figure 11a);
* the seawater temperature annual time series, presented in Section 2.2 (Figure 4b).  
With the above data available, a straightforward calculation of the electricity consumption from the GHPs is executed. The annual electricity consumption time series for the indoor space conditioning with the realization of the proposed passive and active systems is presented in Figure 11b. In Figure 11b,  
Figure 9: Vertical hydraulic diagram of the one out of the two similar open loop geothermal systems.  
By comparing Tables 1 and 3, it is seen that, in total, a saving percentage of 48% on the annual primary energy consumption is achieved, despite the fact of the inadequate indoor space heating in the existing operation and the corresponding 100% coverage with the proposed geothermal system.