## 3 Results  
### Analysis of Efficiency  
A preliminary analysis was carried out in order to examine the effect of the tank capacity \(S\) on the daily efficiency \(E\) and to identify whether the volume of the storage tank was selected correctly. All simulations were repeated for four additional volumes (6, 9, 15, 18 m\({}^{3}\)) in the years 2000-2019, and 2012-2019 for the RWH system, and the RWH system assisted by water recovery from the exhaust air system, respectively. The results of the simulation are presented in Figure 4. Water balance simulations were performed at a daily scale, thus accounting for the effect of extreme rainfall of 24 h duration and dry spells on the RWH system. For any tank size in each system configuration, the daily average \(E\) was computed on the entire analysis period assumed in calculations. The amount of water demand for indoor hydroponic lettuce cultivation was approximately 376.2 m\({}^{3}\) per year. The annual efficiency of the RWH system and the RWH system working with water recovery from the exhaust air system was assessed as an average of the annual \(E\) values related to the analyzed period of twenty years, or eight years, respectively. Focusing on the mean, the average daily \(E\) would grow with tank capacity: For sizes ranging between 6 and 18 m\({}^{3}\), \(E\) varied in the range from 36.8% to 39.8% for the RWH system working individually (Figure 4A), and varied in the range from 87.1% to 92.4% for the combined system (Figure 4B). Although the percentage differences were small, they resulted in saving several cubic meters of water per year and increased the independence of the cultivation from the water supply network. It was more beneficial to choose smaller tanks regarding the economic analysis (see Section 3.2. Cost-Benefit Analysis) while taking into account the ecological analysis on the contrary. The larger tanks provided greater buffer capacity and reduced the amount of discharged water to the sewer. The final decision should be made on the basis of a trade study taking into account all the aspects (economic and ecological analysis, the location of the indoor farm, etc.). The analysis was performed in relation to RI1, RI2, and RI3 for a 12 m\({}^{3}\) tank (as an example size) for both configurations. The results are shown in Figure 5.  
Figure 4: Comparison of the water-saving efficiency with different tank storage capacity for the rainwater harvesting (RWH) system (**A**), and the RWH system cooperating with water recovery from the exhaust air (**B**). As can be seen, the RWH system could only assure total water demand coverage for 29.4% of the days of the analyzed period (RI1, Figure 5A). It could be increased about three times when combining it with water recovery from the exhaust air system (RI1, Figure 5B). The tank supplied a minimum of 10% of the water demand for 50% of days with the singular RWH system, while 96.4% with the combined system (RI2). The biggest difference between the configurations could be seen in the RI3 indicator. The lowest daily water-saving efficiency for the RWH system was incomparably smaller than the combined configuration. The results showed that the RWH system alone allowed us to save water, but it would not be a sufficiently reliable source of water and would not reduce the investment costs of other infrastructure. However, while implementing the combined system, the efficiency increased drastically. This configuration enabled both water savings and investment cost savings, due to the decreased size of external water supply infrastructure. Figure 6A shows a plot of mean monthly demand, rainfall volumes, and yield over the simulation period, while Figure 6B presents the corresponding monthly variation in efficiency \(E\). Water demand for lettuce irrigation was clearly unaffected by monthly and seasonal variations, and showed only slight differences from month to month (on the order of 1 m\({}^{3}\)), while rainfall volumes were seasonal.