Heat-transfer fluids and heat pipes: Efficiency and losses

Why big hot tanks lose little, and ice ships lost so much

What this tab covers How much energy is lost while it moves and while it waits in storage or transit, and how much energy the transport itself consumes, in per cent.

Well-insulated hot stores lose little: Solar Two's molten salt tanks were over 98% efficient per day. Losses rise sharply when heat or cold is held for months or carried far, as the historic ice trade showed. Heat pipes need no pumping energy, but the heat they can carry falls steeply when they are badly oriented or bent.

Chart: Efficiency ranges

Lost while held Lost in transit Whole chain Theoretical limit

Losses by measure

Measure	Range	Type	What is measured
Molten salt two-tank store, daily thermal efficiency (Solar Two)	98 to 99%³	Lost while held	Daily efficiency over 98%; instantaneous efficiency over 99% (1 minus heat loss divided by demand)
Hot salt tank cooling rate	4 C per day³	Lost while held	Temperature drop of the hot tank when idle, allowing storage for weeks
Ice melted, Boston to Calcutta 1833	33 to 44%¹¹	Lost in transit	Tudor's diary reports two-thirds of the cargo intact; the ship's log records about 100 of 180 tonnes arriving (calculated: 80/180) after about four months
CSP round-trip efficiency with next-generation salts	92%⁴	Whole chain	IRENA expectation of over 92% within about five years (2020)
Heat pipe orientation penalty	95%⁷	Theoretical limit	Drop in maximum heat carried between best and worst orientation
Heat pipe bend penalty	2.5%⁷	Theoretical limit	Cut in maximum heat carried per 45 degree bend

Holding periods and inventory

Measure	Value
Molten salt store duration	Average about 7 hours of plant output; hot tank can hold useful heat for weeks²³
Mobile PCM container cycle	About 8 hours to charge and 8 hours to discharge; heat described as carried almost loss-free over 16 km¹⁰
Historic ice voyage	Boston to Calcutta took about four months (12 May to 10 September 1833), with sawdust and wood shavings packed between the blocks¹¹

Losses scale with surface area, temperature difference and time, so large, compact, well-insulated stores lose a smaller share than small ones. Fluids also have hard limits: thermal oil degrades above about 400 C and nitrate salt freezes below about 240 C, so keeping salt molten overnight costs energy. Refrigerant leaks are a climate loss rather than an energy loss and are dealt with in the biosphere tab.

Sources

Molten salt suitable heat transfer fluid for concentrating collector fields solarthermalworld.org, 2021
Final Test and Evaluation Results from the Solar Two Project (SAND2002-0120) Sandia National Laboratories (via UNT Digital Library), 2002
Innovation Outlook: Thermal Energy Storage IRENA, 2020
Design Considerations When Using Heat Pipes (George Meyer) Electronics Cooling, 2016
Waste heat is transported to the customer by truck Bayern Innovativ, 2026
Cold Mine (Susan S. Bean, on the Boston to Calcutta ice trade) American Heritage, 1991