About thermal energy in the sea
When sunshine hits Earth, the energy amount received per second is staggering. The primary absorption mechanism for this energy, is to store it as heat in the ground, the atmosphere and in the ocean. The heat stored in the sea can be used as an energy source, either directly for heating, or by converting into motion and electrical power.
Temperature differences is the engine in the ocean
The ocean waters near the equator obviously get heated more than the polar regions. Thus temperature differences arise, which becomes a driving force of ocean currents. The ocean currents is the big “engine” behind all life in the sea. They transport warm water and nutrients across the world oceans, and makes it possible for life to thrive everywhere.
As noted under “Tidal power”, we can use the ocean currents directly to power electrical generators. However, we can also make use of the ocean heat in different ways.
Heat pumps have become common in many houses, but most of them use the outdoor air as heat reservoir. Those living close to the sea, can instead use the ocean as a heat reservoir, allowing them to get out far more heat than from a common air-to-air heat pump. This is because water has a far higher heat capacity than air, and also because the mean temperature in deep water is far more stable than the air temperature. The air outside is often cold (perhaps -10C or even -20C) in the winter time when heating is needed the most. In the same period, the seawater at 40-50m depth along the Norwegian coast keep a temperature of around +7C.
Many locations are suitable for large heat pumps able to supply district heating to entire towns or city districts. This is an approach used by the company Fjordvarme.
Heat pumps at Runde Environmental Centre
A heat pump using seawater as heat reservoir have been installed at Runde Environmental Centre. The original plan was to take the water from a depth of 40m, thus providing stable and higher water temperatures than the surface layer. However, the cost of a 700m long pipeline meant that using the slightly colder surface water was still the most economical option. The heat pump can supply both Runde Environmental Centre and 8 apartments with heating and pre-heated hot water.
Electricity production from thermal gradients
A heat pump uses electricity to transport heat from one area to another. Typically this involves moving heat from a cold location (outside) to a hot location (living room). Because this is against the natural heat flow direction, additional energy (electricity) is needed. The benefit of heat pumps over direct electrical heating, is that more heat is produced per unit electricity.
When a site have access to both a hot and cold area, a heat pump can be used in “reverse”. It becomes a heat engine, which can turn a thermal gradient into motion and then electricity.
These kinds of heat engines for installation in the sea are called Ocean Thermal Energy Converters (OTEC). They can be installed in areas with warm surface waters and cold deep water. This is typical for deep water in the tropics. There the surface waters frequently keep 25-30C, while the temperature at 1000m depth might only +4C. In these locations it is possible to pump up cold deep water and combine it with warm surface water to power a heat engine based on the Rankine or Stirling principle.
The disadvantage of such plants is that due to the laws of thermodynamics, the efficiency is quite low. However, due to the huge amount of heat stored in the ocean, a considerable amount of electricity can still be produced through OTEC plants.
Pilot plants in exotic locations
OTEC pilot plants have been constructed in Japan and Hawaii, where a considerable research effort is ongoing. OTEC is less relevant in Norway due to smaller temperature differences, but some research on the subject is still ongoing. At Runde Environmental Centre, Lars Golmen is active in this field.