# Seabed Sediment as an Annually Renewable Heat Source
## 1 Introduction  
The limited resources of non-renewable energy, the prevention of climate change and environmental issues like UHI (urban heat island) effect are the concerns that make the search for renewable energy sources important. One of the research topics is geothermal energy. In non-volcanic areas, geothermal energy plays a minor role compared to the radiant energy of the Sun, which has accumulated in the topmost layers of the ground. The term 'geoenergy' consists of both geothermal energy from the Earth and the radiant energy of the Sun. Geoenergy is available, for example, in ground, bedrock, watercourses, in sediment under water bodies and under asphalt and concrete surfaces. In Finland, the most exploited forms of geoenergy are ground source heat and bedrock heat. The new renewable energy sources, such as sediment heat under water bodies and asphalt heat under the surfaces of roads and parking lots, are today attracting great interest. Sediment heat can be harvested from the sediment layers at the bottom of different watercourses. Sediment is a solid-state layer consisting of clay, mud and gytja. There are thick sand layers on the coastline of Finland and plenty of sediment in shallow bays. According to [PERSON] and [PERSON] [1], the temperature variations in overlying water and the thermal properties of the sediment affect the heat budget of the sediments. In their study, a thermistor and a resistance bridge were used in the temperature measurements on sites in Wisconsin. [PERSON] and [PERSON] [2] stated that the thermal effect of the sediments on the lake water temperature is observable in shallow lakes. They also noticed that in seasonally ice-covered lakes the sediment is the major heat source influencing the ecology, e.g., oxygen consumption and nutrients supply. Considering the sea bay conditions, the extraction of heat energy does not appear to be affecting the ecosystem. In 2006, it was observed in Ostrobothnia, Finland, that at a depth of three meters under the water body of lakes the temperature of sediment reached 8 \({}^{\circ}\)C and remained quite stable (8-9 \({}^{\circ}\)C) in deeper layers. In the same year, the interest in renewable seabed sediment heat energy arose in Vaasa in Western Finland too. The location where the heat was first noticed was in Suvilahti Bay. The elevated temperatures were verified by the Geological Survey of Finland, which performed temperature and resistance measurements at three different points on April 11. Following this discovery, the temperatures of sediment were researched as point measurements in many waterways in Ostrobothnia and, according to private communications with the Geological Survey of Finland, it was noticed that there was potential for heat energy at the bottom of lakes and also the sea. Due to previous results, which confirmed the discovery of the promising heat resource, two years later, the low energy network for seabed sediment was introduced in the Vaasa House Fair area of Suvilahti. The Vaasa House Fair area was situated close to the seashore and sediment in the area was found to consist of sulfide clay mud. The long-term average air temperature in Vaasa is \(-5.7\)\({}^{\circ}\)C in winter and \(14.9\)\({}^{\circ}\)C in summer [3]. In our previous papers we have found out that the calculated annual loaded seabed sediment heat energy of 7800 m heat collection pipeline is 575 MWh. Meanwhile the annual extraction of energy in such low energy network is 560 MWh [4]. The goal of the present study is to verify the adequacy of this heat resource for continuous heating of houses. The paper will identify the potential of sediment heat, especially in seabed sediment, using the temperature differences between the lowest and the highest values. Measurements of sediment temperatures were taken in a residential area that utilizes the sediment heat-based low energy network. One focus is to ascertain whether the usage of the low energy network has caused permanent temperature changes in the sediment.