In article , Charlie
Whitaker writes

Second, equip all rolling stock with heat pumps instead of electric
convection heaters (part of the space requirement can be met by the
removal of the heaters). The heat pumps can be used in winter to extract
and concentrate warmth from the outside air and transfer this to the car
interiors (as a marginal benefit, this would cool the tunnels slightly);
conversely in the summer, the heat pumps can be used to provide comfort
cooling. For deep tunnel trains, the heat pumps can be combined with
'coolth reservoirs' (carried under the non-traction cars) so that the
heat pumps can be switched off while the train is below ground; this
avoids the problem of increasingly hot tunnel air reducing heat pump
efficiency. A 'coolth' reservoir uses a compact well-insulated body of
fluid (perhaps a eutectic salt) kept constantly at around freezing
point; energy (latent heat) is transferred into and out of the reservoir
during the solid / fluid phase change. While the train is in the open
(typically at each end of the line) its heat pumps are used to extract
as much energy as possible from the train's 'coolth' reservoirs,
freezing the reservoir fluid. Then, when the train goes underground, it
relies on the stored 'coolth' to continually cool a transfer medium
(water) for use in carriage comfort cooling. The 'coolth' reservoirs
would be sized to yield up all of their latent energy during the tunnel
part of the journey. When the train is back in the open, the process is
repeated.

To keep the cooling load and space requirements to a minimum, multiple
small-scale 'fan-coil' units could be fitted within carriages at high
level; a conservative performance objective might be to cool the top 30%
of carriage air to around 21-23 degrees C. Cool air would circulate
downwards into a less comfort critical zone while becoming warmer; this
vitiated air could then be vented at foot level and, if still cooler
than the ambient external air, used to cool the heat pumps as it is
expelled ('coolth' recovery). The chilled water loop supplying the
fan-coils would be continuous between carriages, since not all cars may
have space to accommodate heat pumps and / or 'coolth' reservoirs.

Given the extent of refitting required, it may be more efficient to
commission new rolling stock. Looking at the state of underground rail
design world-wide, most new air-conditioned rolling stock seems to
consist of established designs fitted with commercial off-the-shelf
cooling units; fully integrated bespoke cooling systems would almost
certainly yield much better space economy and performance.

From a national 'green' energy accounting point of view, the total new
energy load from active cooling could be offset by investing in
renewable source energy generating plant, such as wind turbines or tidal
barrages. It may be possible to make the system carbon neutral.

The financial cost of the system, which, including generating plant and
partial upgrading of the LU power supply, would certainly be
substantial, should also be considered at a national level. Millions of
citizens spend a significant portion of their lives using the tube. It
doesn't matter how much they may or may not be compensated in their jobs
- there is no way that private citizens can bring about a cooler tube.
Only government can do this.

Excellent idea. Have you submitted it on the TfL website? Could be built
into the new stock if the maintenance was easy enough for the depot
crews.
--
Andrew
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