## 4 Conclusions  
Solar energy exploitation in the building sector is a vital way to enhance the renewable share in our society. The goal of the present investigation is to examine various solar thermal systems in the Greek climate zones in energy and economic terms. The most essential conclusions are listed in the next bullets:  
* Higher solar field area leads to higher solar coverage and also to a higher optimum tilt angle for all the solar technologies and locations.
* In the parametric study, it was found that higher efficiency is found with ETC, while the advanced FPC has a bit lower, while the simple FPC has significantly lower performance than the other two choices.
* The minimization of the LCC indicated that the  
\begin{tabular}{l l} CF\({}_{\text{gain}}\) & Yearly economic gain--Cash flow (\(\xi\)) \\ C\({}_{0}\) & Capital cost (\(\xi\)) \\ DD & Dimensionless distance in the multi-objective optimization \\ f & Monthly solar coverage \\ F & Yearly solar coverage \\ F\({}_{\text{R}}\) & Collector heat removal factor \\ F\({}_{\text{R}}\)\({}^{\prime}\) & System heat removal factor \\ G\({}_{\text{T}}\) & Incident solar irradiation on the tilted surface (W/m\({}^{2}\)) \\ H & Daily global solar energy on the horizontal surface (kWh/m\({}^{2}\)) \\ H\({}_{\text{d}}\) & Daily diffuse solar energy on the horizontal surface (kWh/m\({}^{2}\)) \\ H\({}_{\text{T}}\) & Daily global solar energy on the tilted surface (kWh/m\({}^{2}\)) \\ k\({}_{\text{col}}\) & Specific cost of the solar collectors (\(\xi\)/m\({}^{2}\)) \\ k\({}_{\text{el}}\) & Electricity cost (\(\xi\)/kWh\({}_{\text{el}}\)) \\ k\({}_{\text{tank}}\) & Specific cost of the tank (\(\xi\)/m\({}^{3}\)) \\ L & Load energy (kWh) \\ L\({}_{\text{aux}}\) & Auxiliary energy consumption (kWh) \\ L\({}_{\text{u,sol}}\) & Part of the load covered by the sun (kWh) \\ LCC & Life cycle cost of the investment (\(\xi\)) \\ M & Lifetime of the project (years) \\ N & Days of the month \\ r & Discount factor \\ R\({}^{2}\) & Approximation index \\ R\({}_{\text{b}}\) & Ratio of the beam irradiation \\ SPP & Simple Payback Period (years) \\ T\({}_{\text{am}}\) & Ambient temperature (\({}^{\circ}\)C) \\ T\({}_{\text{am,m}}\) & Mean ambient temperature of the month (\({}^{\circ}\)C) \\ T\({}_{\text{cold}}\) & Supply temperature from the grid (\({}^{\circ}\)C) \\ T\({}_{\text{fluid}}\) & Operating fluid temperature (\({}^{\circ}\)C) \\ T\({}_{\text{hot}}\) & Desired temperature of the hot water (= 45 \({}^{\circ}\)C) \\ T\({}_{\text{ref}}\) & Reference temperature (= 100 \({}^{\circ}\)C) \\ T\({}_{\text{w,m}}\) & Mean monthly water temperature from the grid (\({}^{\circ}\)C) \\ U\({}_{\text{L}}\) & Collector thermal loss coefficient (W/m\({}^{2}\)K) \\ V\({}_{\text{tank}}\) & Storage tank volume (m\({}^{3}\)) \\ V\({}_{\text{w}}\) & Daily hot water demand (m\({}^{3}\)/day) \\ X & Parameter for the calculation of the solar coverage \\ Y & Parameter for the calculation of the solar coverage \\
**Greek Symbols** \\ \(\beta\) & Collector tilt angle (\({}^{\circ}\)) \\ \(\beta_{\text{opt}}\) & Optimum collector tilt angle (\({}^{\circ}\)) \\ \(\delta\) & Declination angle (\({}^{\circ}\)) \\ \(\Delta\)t & Time of the month (s) \\ \(\eta_{\text{col}}\) & Collector thermal efficiency \\ \(\rho\) & Density (kg/m\({}^{3}\)) \\ \(\rho_{\text{g}}\) & Ground reflectance \\ (\(\tau\alpha\)) & Optical efficiency \\ (\(\tau\alpha\))\({}_{\text{n}}\) & Optical efficiency for normal incident solar irradiation \\ \(\varphi\) & Location latitude (\({}^{\circ}\)) \\ \(\omega_{\text{s}}\) & Sunset hour angle (\({}^{\circ}\)) \\ \(\omega_{\text{s}}\)\({}^{\prime}\) & Sunset hour angle for the tilted surface (\({}^{\circ}\)) \\
**Subscripts and Superscripts** \\ max & Maximum value \\ min & Minimum values \\
**Abbreviations** \\ DHW & Domestic Hot Water \\ ETC & Evacuated Tube Collectors \\ FPC & Flat Plate Collector \\ \end{tabular}