ef37a wrote:Wonks, indulge an old bottle jockey a while?
Very OT but this brain just thought, there are storage HEATERS but I have never heard of storage coolers?
Same principle but instead of hotness you have a big cold lump. I know of course that "cold" does not radiate but the lump could be positioned such that convection would carry cool air around or some very quiet internal fans?
I am sure you are ahead of my thinking that the cold could be stored with off peak juice and then no noisy AC running in studio time. I also understand that if "I" can think of this, others must have and either kept quiet or found it nonsense. Or of course, maybe thermo-dynamics just doesn't work that way?
First, cold does 'radiate'. As I mentioned in another fairly recent thread, anything above absolute zero radiates energy. It's just that warmer things radiate more energy than colder things, so whilst there is an exchange of energy between hot and cold items, the hot items will always radiate more energy than the cold items (based on the 4th power of the absolute temperature). But if you are next to a cold window in a warm room, you can feel it is cold because the side of your body facing it is radiating warmth away to it, whilst receiving far less heat back from it than other areas of the warm room.
Many offices are now being built with chilled ceilings. Not massively chilled. but maybe down to 18°C. This does a mixture of cooling hot air by convection and also acting as a colder surface seen by your head, and so providing a measure of 'radiative' cooling.
The big problem with storing 'coolth' (as it's known in the HVAC industry) is that of allowable temperature difference. With hot water, you can happily heat it to 80°C, giving the best part of a 60°C temperature difference between it and internal building temperatures (a bit more in say a house where with storage heaters you may only be heating a bedroom to say 17°C).
With cold water, or any other cold substances, you run into the problem that the moment you expose them to air and the temperature of the cold water drops too low, you get condensation, which is generally something you try and avoid. So unless you can fit condensation trays and drains around any form of cold storage heater, you really need to keep the temperature above 10°C (unless you are actively monitoring room conditions and calculating the dew point of the air and keeping the cold setpoint a couple of degrees above the dewpoint temperature, as often happens with chilled ceilings as you don't want rain in the office).
You also really don't want condensation as you loose some of the stored coolth's 'energy' in turning water vapour into liquid ('latent' cooling). It's more efficient to not condense when cooling and keep all the cooling as 'sensible' cooling.
So if we say 10°C as the starting temperature of the cold item, you are left with a temperature difference of only 15°C, given a 25°C room temp. So whatever is used to store the cold needs to be at least 4 times the size of an equivalent heat storage system.
Obviously there are problems with putting large items into rooms that probably need to go against walls and can't have things against them and also weigh a considerable amount. Storage heaters themselves are much larger and bulkier than radiators, so imagine something 4x to 6x larger in each location.
But as the cold storage starts to warm up and the space cool down, a 5°C temperature gain has a much bigger effect on the ability of the cooling system to cool the space down compared to a 5°C loss in a storage heating system to warm the place up. Which really makes it necessary to make the cooling storage system even bulkier so that there's more effective cooling.
Which then leads us on to the method of actually cooling the storage medium down, which in realistic terms, means some sort of chiller system. Yes there are electronic cooling effects (like reverse Peltier effect) that could be used, but unless all your energy comes from renewable sources, you are still trying to minimise electrical power usage, and those methods are electrically inefficient.
Modern modern variable speed chillers with electromagnetic motor bearings are pretty efficient, with a COP (coefficient of performance - ratio of cooling energy out to electrical energy in), often in the region of 4:1 to 6:1. the chiller, like a big fridge, uses a motor and a compression and expansion system to take heat out from one medium (normally water) and transfer it to another (usually air).
There are various things that affect the efficiency of a chiller. Some are:
1. The colder the chilled water leaving temperature is, the less efficient the chiller is.
2. The condenser water/refrigerant gas circuit (taking the heat away from the cooled medium) needs to be kept above a certain temperature, but the hotter it gets above that value, the less efficient the chiller gets. So as the ambient temperature rises, you need to use more cooling fan power to keep those temperatures down.
3. The refrigerant type used in the chiller. Unfortunately the really efficient ones, R11 and R22, turned out to seriously destroy the ozone layer, so are now banned. There have been various blend refrigerants used which are quire efficient, but most of these turned out to have a high greenhouse gas value, so they are being phased out. I'm now a few years out of date, but ammonia was probably the main remaining contender for large chillers, but as a gas it's deadly in not too high a concentration. Chillers will leak refrigerant slowly over time, so good ventilation is essential for ammonia chillers, but if the condenser cooling system goes horribly wrong (and it can do) then gas pressures in the chiller can go beyond the safety point and the chiller equivalent of a safety valve - a 'bursting disk' should rupture and let the gas escape to atmosphere, not something you want to do with a large quantity of hot ammonia gas. So really not viable for many situations.
So you've got some benefit in having a chiller produce water at 8°C (to cool a medium down to 10°C) compared to a more normal 6°C.
But you would end up having to pipe a lot or water round in insulated pipes, running secondary pumping systems, which all becomes rather expensive for a not very efficient form of cooling system. Its a lot simpler to install DX (direct expansion) or VRV (variable refrigerant volume) system in small spaces and small offices, and full AC with maybe FCUs (fan coil units) on walls or in ceilings in larger offices. These can provide both heating and cooling functions.
There was a form of cooling storage used on main chilled water systems for buildings that was quite popular in the very late 80s and early 90s, called 'ice storage'. It came about when there were cheap off-peak electricity tariffs available. By storing coolth in the form of ice, and creating this at night when it was cheaper to run the chillers, the stored coolth could be used to 'peak lop' the cooling demand on the chillers during the day, meaning that the chillers themselves didn't have to be so large - capital and operational savings. The ice was stored within sealed plastic balls in a large storage tank.
There were several drawbacks:
1. To store a reasonable amount of coolth in the form of ice, you need to have a lot of storage capacity. The delta-T in the system is small (normally 0°C to 6°C) so you need to have a lot of balls. Yes, you benefit from the stored phase change energy from turning 0°C ice into 0°C water, which helps. But it still wasn't a compact means of storage. You needed a large well insulated tank, which will weigh many tonnes and you need a place to put it in, which normally meant a large basement plantroom. Which meant that you had to have a building that was big enough to have such a space to spare.
2. To create ice, you need a cooling medium that's below 0°C, ideally several degrees below if you want to create it within a reasonable time period (the night tariff didn't last for ever. 7 hours maximum IIRC, though sometimes it was less depending on the amount of electricity taken. Which means that you can't use standard water as the cooling medium or your chiller freezes. So you need to have a fairly high anti-freeze content in the cooling water. This reduces the specific heat capacity of the water, so for an air handling unit cooling coil to produce the same kW of cooling, it needs larger coils and a higher water flow rate. So pumps and pipework all go up a size or two.
3. The chiller setpoint need to be set down to at least -3°C/-4°C, instead of the typical 6°C. This makes the chiller work a lot harder, it's efficiency drops and whilst it uses cheap electricity, it's using more than it would normally. Slightly offset by cooler night time temperatures making it easier to cool the chiller's condenser circuit.
4. You need a very complicated control system to operate a) the ice making and then b) the switch back to normal operation and then c) the blending in of 0°C water with warmer water from the chillers (say at 7°C to get to a 6°C common flow temp). It has to decide when the storage system is full so you don't need to charge it any more, and also when the storage system is basically empty, so you stop passing warm return water (that hasn't gone through the chiller) through it. It also needs to be aware if the storage system hasn't charged (due to a faulty motorised valve). The storage tanks came with their own %storage full output signal to the BMS (building management system) but these rarely worked well so we stopped using them for control and added our own sensors. There were a lot of possible failure scenarios and a practical inability to test the system fully before handover to the client.
Towards the end of the period (and occasionally later when a mech. engineer reinvented the wheel), there was a move to utilise a different phase/change material, which changed from liquid to solid at a lower temperature than ice. This gave a more compact storage solution for the same storage capacity, but meant that the chillers had to operate at even lower temperatures, which really limited the choice of refrigerants as not all could create say -8°C flow temperatures.