# Seabed Sediment as an Annually Renewable Heat Source
## 2 Thermal Interaction between Water and Sediment  
The use of sediment thermal energy for heating is based on the annual recharge of the thermal energy in sediments. This recharging is mainly caused by an interaction between water and sediment. Of course, water and sediment interact in many ways. Here, the focus is on the thermal interaction when water is either brine or fresh water. The main feature affecting the thermal energy of sediments is the temperature of the overlaying body of water when there is no tide. [PERSON] et al. [5] carried out one of the first studies of this feature as early as 1927 at Lake Mendota where the heat exchange of mud was measured at different water depths but with sediments within the thickness of five meters. [PERSON] et al. noted a variation in the migration rate of the heat. They showed that the annual oscillation at the sediment temperature attenuates as a function of increasing depth. In a study conducted at Indian River on the Atlantic Coast of Florida in the 1980s, [PERSON] [6] also reported results that show that the water column and underlying sediments have an active exchange of heat. Many publications indicate that the heat exchange between the water body and the lake sediments cannot be ignored, especially when a lake is shallow or has an ice cover for some period of the year [7; 8; 9; 10]. The role of sediment in transferring the stored heat back to the water body during winter has been investigated in the arctic lakes, for example by [PERSON] and [PERSON] [9]. According to [PERSON] [11], the heat flow from the sediments happened during the whole of winter but the convective mixing was very slow. The sediment heat flux is a signification parameter when simulating the stratification conditions in shallow lakes but not in deeper lakes [10]. A study of dynamic heat exchange in the sediment-water interface of a lake indicates the same conclusion: [PERSON] and [PERSON] [12] noted that the sediment heat fluxes are a very important factor in shallow lakes. The lake sediments provide seasonal heat storage and thermal inertia to water columns. The direction of the heat flux has both seasonal and daily variations. Seasonal variation causes heat transfer from water to sediment in the summer and from sediment to water in the winter. [PERSON] [13] studied shallow lakes with depths approximately between 0.5-1.0 m and 1.8-2.7 m in the Barrow area of Alaska. He stated that during the ice-free period, the water in the lake can be considered to be in an isothermal state with a maximum recorded temperature of 12 \({}^{\circ}\)C. Following the icing of the lake surface, the heat radiation through the ice can quickly warm the water, even from approximately 0 \({}^{\circ}\)C to 2 \({}^{\circ}\)C, as long as the ice is not covered by snow. The snow on the ice causes the cooling of the water and then the bottom sediments start to warm the water. Warming depends on the distance from the shore and the depth of the water body. A great deal of heat energy is available in urban areas. Buildings and surface materials, such as concrete and asphalt pavements, absorb and store sunlight effectively; together, the built environment, people, vehicles and lack of flora are the reasons for the extra heat [14]. This phenomenon, referred to as the urban heat island (UHI) effect, is largely noticed in big cities. The air in an urban area is warmer than that in the nearby rural area. The difference may even be several degrees [15]. In Turku, Finland, the average UHI intensity has been measured as 1.9 \({}^{\circ}\)C and the periodic difference between temperatures in the city center and surrounding areas have risen up to 10 \({}^{\circ}\)C [16]. The UHI effect also has an influence on ground and water temperatures. There are many different kinds of water courses, bottom sediment and heat beneath urban areas and these can be utilized as low energy sources for buildings. In their research, [PERSON], [PERSON] and [PERSON] concluded that the urban heat island effect feeds itself [17].