## 4 Active Systems
### Hot Water Production and Swimming Pools Heating
#### 4.2.3 Proposed Passive and Active Systems for Hot Water Production and Swimming Pools Heating  
The swimming pools heating loads can be reduced with the placement on their upper surfaces of an insulating, floating cover, during the time periods they are not used. Due to this passive measure, in the proposed operation, the U-factor regarding the heat losses from the swimming pools free surface to the indoor ambient environment will either take the values already calculated in Section 4.2.1, whenever the swimming pools are in use, hence no insulating cover is placed on their surfaces, or the values calculated with the process presented below. Specifically, when the swimming pools are not in used and the insulating floating cover is placed on their surfaces, heat will be transferred from the water to the insulating floating cover with convection, then through the cover with conduction and finally from the cover to the ambient environment with convection and radiation. The U-factor regarding the overall heat transfer process is given by the relationship:  
\[\mathrm{U}_{\mathrm{c}}=\frac{1}{\frac{1}{\mathrm{h}_{w}}+\frac{\mathrm{d}}{ \mathrm{k}}+\frac{1}{\mathrm{h}_{\mathrm{c}}}}, \tag{9}\]  
where:  
* \(\mathrm{h}_{\mathrm{w}}\) = 50 W/(m\({}^{2}\)K) is the convective heat transfer factor of still water [52; 54];
* \(\mathrm{d}\) = 2 cm is the floating insulating cover thickness, given by certified manufacturer;
* \(\mathrm{k}\) = 0.025 W/(m-K) is the floating insulating cover conduction coefficient, given by the manufacturer; the material of the insulating cover is polyethylene;
* \(\mathrm{h}_{\mathrm{fc}}\) = \(\mathrm{h}_{\mathrm{rc}}\) + \(\mathrm{h}_{\mathrm{c}}\) is the total heat transfer factor from the insulating cover to the ambient, equal to the sum of the convective heat transfer factor \(\mathrm{h}_{\mathrm{c}}\) = 2.5 W/(m\({}^{2}\)\(\cdot\)K) for indoor space air flows above horizontal surfaces and the radiation heat transfer factor \(\mathrm{h}_{\mathrm{rc}}\) of the insulating material; the latter is given by a relationship similar to Label (5), by replacing the emissivity of water with the emissivity of the insulating material \(\mathrm{\varepsilon}_{\mathrm{c}}\) = 0.55, as provided by a certified manufacturer. Given the introduced passive measure, the swimming pools heating loads are calculated again. Their annual fluctuation is depicted in Figure 14a. The direct impact of the insulating floating cover is the reduction of the annual thermal energy demand for the swimming pools heating at 54,620 kWh. This means that a 51.3% reduction percentage is achieved, compared to the existing annual thermal energy demand (112,216 kWh). For thermal power production, a solar-combi system is proposed. Solar-combi systems are practically hybrid thermal plants, aiming at the maximization of the thermal power production from the R.E.S. unit, namely the solar collectors, with the support of thermal storage tanks. In the present study, water thermal storage tanks are utilized. The system is integrated with a biomass heater [56]. In the Pancretan Stadium, four new thermal storage tanks will be introduced, replacing the existing ones, due to their ageing, with double hydraulic inlet for concurrent thermal power storage from both the solar collectors' field and the biomass heater. Each thermal storage tank will be connected to a particular solar collectors' group, formulating, in this way, four independent solar-combi systems. Thermal power inlets from the biomass heater will be connected to all four thermal storage tanks, supplementing the thermal power production from the solar collectors. Each thermal storage tank will serve a particular, neighboring part of the overall hot water distribution system. The essential theory on solar collectors' thermal processes and thermal storage processes is applied for the dimensioning of the introduced solar-combi system [57].