**3. Results**

#### **3.1 Results for our previous design**

Let us summarize now the major results obtained by using our previous design, which is based on the present design developed here. So, that design is similar to the present design, but has some minor differences:


This design was performed in our previous work [1]. Summarizing, the analysis performed had found several different behaviors:


The calculation of any solution implies choosing a tank size (*M*) and calculating the minimal number of solar collectors (*N*) needed in order to satisfy the space heating demand, that is, that the water temperature of the tank works always within the usable range (33–85°C) in order to provide the space heating demand. However, in this procedure, we must keep in mind that the dynamical model considers only average parameters (temperatures, etc.). So, during very cold days and especially when very small tanks are chosen, this calculation could not be conservative, since the thermal storage capacity of the water tank could not be enough for overpassing such an event. Therefore, let us study now the behavior of the STES system during the worst weather event (having several fully cloudy days) that we will define as a tenday cloudy-weather event. This event is calculated in our model by means of not considering the average monthly solar irradiance, and instead considering the collector's yield obtained during cloudy days. So, we are calculating the (higher) number of collectors needed for solving this extreme condition. These N numbers are illustrated in **Table 2** (in brackets) for φ = 78°, together with the usual average solution. Here is observed that this very cold winter leads to very poor performances for small tanks, which must be supported by using much more collectors. Otherwise, large tanks can easily manage this scenario; for example, case **A** (*M* = 170 m<sup>3</sup> ) provides enough storage for one month, and so, the number of collectors are the same in both (average and worst) cases.

**Table 3** summarizes the breakdown of cost (for the case φ = 78° and ten-day storage capacity) for the previous cases studied in our previous work [20]. It is interesting to note that the larger tank (case **A**) obtains a total cost slightly higher than the other ones, but it provides the largest storing capacity (one month) that can fully provide the heating demand. However, in this case, it should also be evaluated the visual impact of placing this large aboveground tank close to the house. So, let us study now another option for providing this large storage capacity, which is performed by using the next smaller tank (case **B**) with eighteen solar collectors, so getting a total cost of €32,200.

**Table 4** repeats the previous analysis by using flat solar collectors (STES\_Okotoks-Flat collectors, in [20]) having each one the same solar area (2.088 m2 ) as the previous vacuum-tube collectors, but, of course, they both have different efficiency curves,


#### **Table 2.**

*(N, M) solutions for vacuum-tube collectors.*


#### **Table 3.**

*Breakdown of costs for different solutions (*φ *= 78°).*

*Holistic and Affordable Approach to Supporting the Sustainability of Family Houses… DOI: http://dx.doi.org/10.5772/intechopen.103110*


#### **Table 4.**

*(M, N) solutions for flat collectors (tilt angles 78° or 45°).*

according to **Figure 1**. The performance of these flat collectors has been simulated (following our previous discussion) by reducing 25% the solar factors *α<sup>n</sup>* x*G<sup>n</sup>* previously calculated for vacuum-tube collectors. Similarly, the *In* fluxes previously calculated are now 25% reduced and then used in the efficiency equation (Eq. (9)).

Now, by comparing **Tables 2** and **4**, we can observe that flat collectors always get lower efficiencies than vacuum-tubes ones and that their efficiency decreases strongly as much as the tank size is reduced, and so, their working temperature is increased. Another difference regarding vacuum-tube collectors is that flat collectors achieve negligible efficiencies during winter and so, their performance is highly penalized when small tanks are used. Otherwise, it is interesting to note that their performances are very reasonable by using large tanks, in which they can take advantage of their higher efficiencies during summer (**Table 4**).

In order to estimate the total cost of these alternative designs for Okotoks, it will be assumed that the unitary cost of flat collectors is equal to previous vacuum-tube ones, regarding that there is a wide range of commercial models for both kinds of collectors and so, different cost choices. **Table 5** shows the total investments for the previously studied cases. Here is can be observed a different behavior regarding the previous systems with vacuum-tube collectors, since now the best choice is always obtained with the largest tank (case **A**). In addition, this large tank can support the collectors installed on different tilt angles.

It is interesting to compare this case (solar area 37 m<sup>2</sup> ) with the Okotoks' project that uses a similar collector's area (44 m<sup>2</sup> per house). This 170 m<sup>3</sup> tank gets an average efficiency of 82%, which is higher than the Okotoks STES efficiency (60%), due to their lower size and higher thermal insulation (the Okotoks reservoir uses 583 m<sup>3</sup> of rocky underground per house). The comparison with the Friedrichshafen's project is also interesting since both use water tanks as STES system. Here is observed that the German project uses a larger tank (320 m<sup>3</sup> ) with low efficiency (60%) and a higher


**Table 5.** *Total costs for flat collectors.*

solar area (108 m<sup>2</sup> ) too, leading to a noticeable higher overall cost (128,000 U\$) per equivalent Okotoks' house.

Finally, **Table 5** is presented the total cost of these cases for underground tanks, calculated from the Galway's underground tank (€17,600). Here we can observe that an underground tank always leads to a remarkable higher cost than the aboveground option. Hence, since a small tank can be conveniently be insulated, we will prefer aboveground tanks. Moreover, as we will discuss in the next section, this aboveground could be installed within the greenhouse near the house, and this way, its heat losses can be useful for warming the greenhouse.
