The idea of moist air as a heat transfer fluid in energy-efficient home heating

by Stephen Hewitt | Published 1 July 2022

Could using moist air as a heat transfer fluid increase energy efficiency in the design of a domestic central heating system or solar heating system? This theoretical article presents the general idea of using moist air as a heat transfer fluid and illustrates reasons for thinking it might be useful with two specific untested ideas.

The general potential of moist air as a heat transfer fluid

The amount of water vapour carried by air at 100% humidity increases exponentially with temperature. At 20°C a cubic metre of saturated air would contain about 17g of water. At 60°C it would contain 130g of water. [TOOLBOX]

Transferring heat with moist air can be explained by saying that when saturated air is cooled, water condenses out, providing its latent heat of evaporation. The latent heat of evaporation of water is about 2.6 MJ/kg while its specific heat is about 4.2kJ/kg. The first figure is three orders of magnitude more than the latter, which is why the comparatively small amounts of water in moist air are still interesting for heat transport.

Table 1 shows a comparison of a calculated approximate cost in power to transfer heat down some form of pipe for three different heat transfer fluids: water, humid air and dry air. The heat transferred is based on the stated temperature change, so for example the two temperatures might correspond to the “flow” temperature of water leaving a central heating boiler and the return temperature.

The figures for flow rate and power consumed are taken from representative product specifications on the internet in June 2022. The water pump is a central heating circulating pump [STUART]. The air figures are for a duct fan [VENTAXIA]. The figures for air and water are not necessarily strictly comparable in the sense that no attempt has been made here to equate the length of duct through which this fan could pump the air with the length of water pipe through which the pump could push the water.

Regardless of this, one conclusion from these figures is that the cost in energy of pumping hot water is negligible. The cost of pumping humid air is an order of magnitude more than water for the 60 - 40 °C temperature transition but is still less than 1% of the heat transferred. At higher temperatures the cost will be lower because of the exponential increase in the concentration of water vapour. Solar collectors could reach such temperatures, without reflectors. The cost of moving heat in dry air is nearly an order of magnitude more than moist air at 60 °C .

In addition to the feasibility of pumping moist air through ducts, water vapour has the potential to enhance thermo-syphon or natural convection of air in systems that use it, including possibly solar collectors. This is because the water molecule is lighter than than both the oxygen and the nitrogen molecule. The more water a volume of air contains, the lighter it will be. So if the air is always saturated it will become less dense with temperature for two reasons. One is the usual reason of the gas expanding in proportion to absolute temperature and the second reason is the exponential increase in the amount of water in saturated air with temperature. And wet air will be lighter than dry air at the same temperature and pressure.

An idea for using moist air in a central heating system

Figure 1 shows the idea of making a radiator like a plastic balloon or bag and using moist air inside it to transfer heat to its thin walls. A slight variation on this might be a rigid plastic radiator. Figure 2 shows schematically the same idea extended with moist air used for transport of heat round the house and not only within the radiator. The motivations for this will be explained in relation to the following general points about heating.

Low temperature heating

A home central heating system that uses water at a lower temperature than traditional systems can be desirable for several reasons.

• A heat pump will produce heat more efficiently at lower temperatures
• A solar heating panel for lower temperatures will be cheaper.
• If heat is stored, for example in a large water tank, then lower temperatures require less insulation

A lower water temperature means that the radiator emits less heat. A larger surface area of radiator can compensate for this.

Thermal inertia

The instantaneous rate of heat loss from a house increases monotonically with the difference between its inside temperature and the outside temperature. To reduce the average rate of heat loss and hence heating costs, ideally the house would go cold immediately you left it and become warm again immediately you returned.

For the same reason it is desirable that the heating system itself has a low thermal inertia. Ideally it would make you warm as soon as you turn it on and stop producing heat as soon as you turn it off.

The advantages of radiant heating

Radiant heat directed at you, like the sun coming out, can make you feel warmer without necessarily heating up the room. It might allow the same level of comfort with a lower air temperature, thus reducing heat losses from the building.

A British government web page on personal comfort: “Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment.” [HSE]

Radiant heating also has the advantage that you feel it instantly. In a sense heating the air instead is like putting the heat through an extra heat exchange where heat first has to be exchanged to the air (and dry air is not good at carrying heat) and then further transmitted to you.

However the word “radiator” is a misnomer. These things emit more heat by convection than they do by radiation. To emit enough heat by radiation alone the surface area would have to be larger.

How humid air might help

Based on these premises, some motivations for the idea of plastic radiators and moist air are as follows.

• The radiator is cheap, so a large total surface area of radiators will be cheaper. A large surface area would help with the lower radiator output of low temperature heating or reduced convection output of radiant heating.
• The radiator is lightweight, both because it is made of thin plastic and because it does not contain water, making it cheaper to mount on the wall or ceiling.
• A rigid plastic radiator might cover an entire ceiling, providing a large surface area for radiant heating.
• In particular mounting a radiator on the ceiling might help to prevent convection and increase the relative amount of heat emitted by radiation (assuming insulation that prevents heat loss through the ceiling).
• It may be possible to make the walls of the radiator very thin because they do not have to withstand water pressure.
• If the radiator is flexible that might allow novel installation options such as bending it round corners.
• Flexible radiators might fold away when not in use, for example in the summer.
• A variation would be a portable radiator that has to be filled in batches like a hot water bottle.
• The radiator itself has low thermal inertia and will start to emit heat as soon as hot moist air arrives inside it.
• The thermal inertia of the entire heating system is less than the traditional one with pipes filled with water.

There are many potential obstacles and reasons why this might not work, including the following.

• Noise from the fan which might be transmitted through the thin radiator walls.
• The same low water temperature which make a large surface area of radiators desirable also make moist air less efficient as a heat transfer fluid, because of the exponential increase in water content with temperature.
• The evaporator is effectively an extra heat exchange stage which will impose an extra temperature drop.
• There could be safety concerns with bacteria growing in a hot water store or the radiators themselves, since they can be damaged and leak to the room much more easily than a steel radiator.
• A way needs to be found to drain the condensed water from the radiator. In particular with flexible thin radiators mounted horizontally on the ceiling it might collect in pools and not be easy to drain.

An idea for using moist air in simple solar collector

References

[HSE]

https://www.hse.gov.uk/temperature/thermal/factors.htm

in June 2022, this Health and Safety Executive page titled “The six basic factors” started: “The most commonly used indicator of thermal comfort is air temperature - it is easy to use and most people can relate to it. However, air temperature alone is not a valid or accurate indicator of thermal comfort ”...

[STUART]

https://www.stuartplumbing.co.uk/lowara-ecocirc-25-6-130-circulating-pump/

In June 2022 this plumbing supplier's web page claimed “Delivery: up to 3.2 mm³/h” and “Power: from 10 W up to 50 W” for the Lowara Ecocirc 25-6/130 Circulating Pump.

[TOOLBOX]

https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

In June 2022 this web page included the values in table 2, below.

[VENTAXIA]

This fan for a 150mm duct diameter moves 70 l/s at a pressure of 200Pa and uses 0.1kW according to the table on page 3 of the PDF at https://www.vent-axia.com/file/196511/download?token=4AkKYvam in June 2022. (SHA256 hash 228c1bb114c1b1a61f610d8e95a07363a22a50e0e077972dccb4dfc50101f52d) There was a link to this from https://www.vent-axia.com/range/powerflow-acp-line-duct-fans