Low ΔT heat

Low ΔT heat describes a hot heat source and a corresponding cold heat sink which differ in temperature by a relatively small amount, such as 40 kelvins (&deg;C).

The sun delivers 89 million gigawatts to the land and water of the earth. That same energy is re-radiated out into space. See Earth's energy budget, which states:
 * Total power entering the Earth's atmosphere is about 174 petawatts.
 * 174 petawatts is 174 million gigawatts or 174 billion megawatts.
 * 70% is absorbed and later re-radiated: 51% by land and water, the other 19% by air.
 * 51% of 174 petawatts is 89 million gigawatts.
 * 19% of 174 petawatts is 33 million gigawatts.
 * 38% eventually arrives in the atmosphere and is re-radiated.
 * 38% of 174 petawatts is 66 million gigawatts.

Thus, the atmosphere receives 66 million gigawatts over 24 hours, and eventually re-radiates all of it. That comes to 5713 billion gigajoules of energy received over a day. There is a lot of energy flowing through the atmosphere at ambient temperature.

A heat sink which is colder than ambient will absorb heat from the atmosphere. This heat can be moved from the heat sink upward into a large heat reservoir using gravity feed heat pipes to carry heat upward while not carrying heat out of the reservoir when the heat sink is colder than the reservoir.

Similarly, when the heat sink is warmer than ambient, it will radiate heat into the atmosphere. This heat is replaced from the cold temperature reservoir using gravity feed heat pipes to carry heat upward while not carrying heat back into the cold reservoir when the heat sink is warmer than the cold reservoir.

By building two large insulated container of high heat-capacity material such as salt or sand, with gravity feed heat pipes ready to carry heat into but not out of the hot reservoir, and out of but not into the cold reservoir, a heat engine can operate between the two reservoirs at all times or as needed, without any energy input except from ambient. Whenever solar energy is available, it can be fed into the hot reservoir and saved for use as needed.