# Seabed Sediment as an Annually Renewable Heat Source
## 4 Results  
According to the initial measurement period in 2008-2009, the cooling of the sediment took place until February 2009, when the lowest temperatures were measured. Sediment temperatures were very close together in December and January. The sediment temperatures for a six-month period in 2008-2009 are shown in Figure 2 ([PERSON], [PERSON]; [PERSON] Geological Survey of Finland. Personal communication, 2009). This was the first season in which the houses were heated using the low energy network. The sediment temperatures were presented from a distance of 50 to 300 m from the shore. This distance was chosen because 0-50 m is relatively close to the shore and, therefore, flora (reeds, etc.) might affect the temperatures. Figure 1: (**a**) Profile of the heat collector pipe and the location of an optical cable; (**b**) The assembling of heat collection pipes. The mean air temperature in Ostrobothnia in the year 2008 was over 5.0 \({}^{\circ}\)C and in the year 2009, it was 4.0-5.0 \({}^{\circ}\)C [22]. The monthly mean air temperature in Vaasa is shown in Table 1 ([PERSON] Finnish Meteorological Institute. Personal Communication, 2017). The heating season began in October, which can be observed as a decrease in sediment temperatures in November 2008. Further, the mean air temperatures were already dropping when the winter season arrived. A high and significant correlation has been observed between the air temperature and the temperature of sediment after one or two months [21]. The value of the maximum sediment temperature per month (Table 1) was calculated as an average of temperatures at a distance of 280-300 m from the shore ([PERSON]; [PERSON] Geological Survey of Finland. Personal communication, 2009). The temperatures of the geothermal fluid were measured for the period November 2008 to February 2009 in one house connected to the low energy network. The relationship between the temperatures of the air, sediment and geothermal fluid from September 2008 to February 2009 are shown in Figure 3. In December, the geothermal fluid was measured as slightly warmer than the sediment, which can be explained by the different measurement locations. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Month**} & \multicolumn{2}{c}{**t\({}_{\text{ka\_air}}\)[\({}^{\circ}\)C]**} & \multicolumn{2}{c}{**t\({}_{\text{max\_sed}}\)[\({}^{\circ}\)C]**} & \multicolumn{2}{c}{**t\({}_{\text{ka\_fluid}}\)[\({}^{\circ}\)C]**} \\ \cline{3-5}  & & **Air** & **Sediment** & **Geothermal Fluid** \\ \hline \multirow{3}{*}{**2008**} & September & \(9.3\) & \(8.1\) & - \\  & October & \(6.7\) & \(7.0\) & - \\ \cline{1-1}  & November & \(1.8\) & \(3.9\) & \(4.0\) \\ \cline{1-1}  & December & \(0.2\) & \(0.4\) & \(2.0\) \\ \cline{1-1}  & January & \(-3.6\) & \(0\) & \(0.0\) \\ \cline{1-1}  & February & \(-6.1\) & \(-1.2\) & \(0.0\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The monthly mean air temperatures (t\({}_{\text{ka\_air}}\)) and the monthly maximum sediment temperatures (t\({}_{\text{max\_sed}}\)) and the geothermal fluid temperatures (t\({}_{\text{ka\_fluid}}\)) in Vaasa during the measurement period in 2008–2009. Figure 2: Seabed sediment temperatures measured from September 2008 to February 2009 in Lito-oravankatu. The loading of sediment heat is shown in Figure 4. The temperatures of the sediment increased between March and August up to 11-12 \({}^{\circ}\)C. By May, the ambient air had warmed up and the snow and ice had melted. The water was becoming warmer and the heat in the sediment layer was also increasing. Another great difference in sediment temperatures occurred between June and July. This indicates the end of the house-heating season. The heat loading in the sediment layer starts from the shore and goes to the end of the pipe, which can be observed as a constant curve without any slope in July and August. Figure 4: Seabed sediment temperatures measured from March 2009 to August 2009 in Litio-oravankatu. The Sun is obviously loading the heat to the seabed sediment.