# Seabed Sediment as an Annually Renewable Heat Source
## 5 Discussion  
Different models of interaction between water and sediment has been studied e.g., in North-Western Russia and Germany, [2] and in United States [10; 23]. The measured data for all the periods--2008-2009, 2013-2014 and 2014-2015--show the cooling of sediment during the autumn and winter seasons. The difference in sediment temperatures between a month with the highest temperature value and a month with the lowest temperature value in each period varied from 9.7 to 11.2 \({}^{\circ}\)C. The level of the temperatures was about 4 \({}^{\circ}\)C lower in the early stage of the new low energy network (2008-2009) than it was five years later. In the first measurement period, 2008-2009, measurements were taken with a device that was different to that used in the follow-up measurement period of 2013-2015. It is also true that because of heating and the drying of the constructions, the houses needed more energy during the first heating season than they did in later years. This has had a decreasing influence on the temperatures of the sediment.  
There is a roughly 2 \({}^{\circ}\)C difference between the temperatures measured near the shore and those measured at a distance of 300 m from the shore. This difference may reflect the depth of the water layer over the sediment. The farther from the shore the measurement is taken, the thicker the water layer and the warmer the sediment. Accurate data about the depth of the water layer are not available for the Suvilahti area. On the other hand, the position of the cable is estimated to be 3-4 m under the seabed but the exact position of the cable at the different distances from the shore cannot be measured.  
In the measurement period 2008-2009, the heat carrier fluid temperature was higher than the highest sediment temperature each month in that period. This is explained by the accuracy of the Ouman fluid temperature meter, which was \(\pm 1\) degree. The result is intelligible if the limit of error is taken into account. The other factor that may have had an effect on the comparability of the fluid and sediment temperatures is that the measurements of the heat carrier fluid were made in the engineering  
Figure 11: The between-month difference in sediment temperatures for the months with the highest and the lowest temperature values in the periods 2008–2009, 2013-2014 and 2014-2015. The polynomials of second degree are drawn as trend lines.  
and utility service room inside a house, whereas the sediment temperature measurements were taken next to the collection well on the shore. It may also be possible that the surroundings of the collection pipe in the seabed sediment were frozen and that ice cover may have acted as an insulator for the heat carrier fluid.  
The validity of the sediment temperature data is somewhat questionable because of the location of the temperature measurement cable, i.e., connected to the side of the operating heat collection pipe that contains the heat carrier fluid. On the other hand, the influence of the fluid can be expected to be quite small. In future studies, to gain more accurate sediment temperatures, it would be good to attach the temperature measurement cable to bare sediment in the same location and to compare those temperatures with the current results.  
The possible flow of groundwater should be known when estimating the heat capacity of a particular area of the sediment. These data are missing for Suvilahti.  
Ice and snow cover have an effect on the temperatures of water, which, according to [PERSON] et al. [6] interacts with the sediment. There are no data available about the thickness of the ice or the depth of snow cover in Suvilahti. The stationary stage of sediment temperatures can be observed in a particular section of every study period and this reflects the impact of ice and snow cover. The heat transfer occurs farther from the pipeline and a stationary stage prevails close to the pipeline.