# Seabed Sediment as an Annually Renewable Heat Source
## 4 Results
Figure 3: The relationship between the air, sediment and geothermal fluid temperatures in the low energy network of Suvilahti, Vaasa, 2008–2009. The lines are drawn to visualize and approximate relationship between measurements points. In the follow-up study of 2013-2014, the sediment temperature cooled down until January 2014 (Figure 5). The heating of houses began in October, which can be seen in the great sediment temperature difference between October and November. The mean air temperature in Ostrobothnia, Finland in 2013 was 5-6 \({}^{\circ}\)C [22]. The monthly mean air temperatures in Vaasa for a five-month period in 2013-2014 are presented in Table 2 ([PERSON] Finnish Meteorological Institute. Personal Communication, 2017). During the measurement period of 2013-2014, it can be noticed that, on average, the sediment temperatures were higher than the air temperatures in every month except September, when they were almost equal. The relationship between the temperatures of the air, sediment and geothermal fluid in the period September 2013 to January 2014 are shown in Figure 6. The temperatures of the geothermal fluid were cooler than the sediment temperatures for the whole measurement period of 2013-2014. The warming of the sediment layer from spring to late summer 2014 is shown in Figure 7. The temperatures of the sediment were increasing by up to 14 \({}^{\circ}\)C from March to August. The greatest difference in sediment temperatures is between April and May. The heat loading in the sediment layer takes place along the whole area from the shore to the distance of 300 m, which can be observed as a constant curve without any slope in July and August. The difference in temperatures in July and August is about 3.5 \({}^{\circ}\)C. The effect of the Sun is obvious. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Month**} & \multicolumn{2}{c}{**\(\mathbf{t_{ka\_air}}\)[\({}^{\circ}\)C]**} & \multicolumn{2}{c}{**\(\mathbf{t_{max\_seed}}\)[\({}^{\circ}\)C]**} & \multicolumn{2}{c}{**\(\mathbf{t_{ka\_fluid}}\)[\({}^{\circ}\)C]**} \\ \cline{3-5}  & & **Air** & **Sediment** & **Geothermal Fluid** \\ \hline
**2013** & September & 11.9 & 11.7 & 10.0 \\  & October & 6.3 & 10.2 & 7.0 \\  & November & 2.6 & 5.2 & 5.0 \\  & December & 0.8 & 3.1 & 2.0 \\
**2014** & January & \(-\)8.0 & 1.6 & 0.0 \\ \hline \end{tabular}
\end{table} 
Table 2: The monthly mean air temperature (\(\mathbf{t_{ka\_air}}\)), the maximum sediment temperatures (\(\mathbf{t_{max\_seed}}\)) and the geothermal fluid temperatures (\(\mathbf{t_{ka\_fluid}}\)) in Vaasa in the measurement period 2013–2014. Figure 5: Seabed sediment temperatures measured from September 2013 to January 2014 in Lito-oravankatu. In the final follow-up study, 2014-2015, the sediment was cooling down until February 2015 (Figure 8). The great gap between the temperatures measured in September and October is due to the start of the house-heating season. Exceptionally, the sediment in December was warmer than in November. The autumn of 2014 was unusual in terms of sea water and air temperatures [22]. Energy consumption was minor in November and the heat was gathering close to the collection pipe. Figure 6: Temperatures of air, sediment and geothermal fluid in measurement period 2013–2014. Figure 7: Seabed sediment temperatures measured from March 2014 to August 2014 in Liito-oravankatu. Temperatures increase after the winter months. The mean air temperature in Ostrobothnia, Finland in 2014 was 6.1 \({}^{\circ}\)C and in 2015 it was 6.4 \({}^{\circ}\)C ([PERSON] Finnish Meteorological Institute. Personal Communication, 2017). The monthly mean air temperatures in Vaasa in a six-month period in 2014-2015 are presented in Table 3. In the measurement period 2014-2015, it can be seen that, on average, in every month, the sediment temperatures were higher than the air temperatures. Further, the sediment temperatures were higher than the geothermal fluid temperatures for the whole period.