Posted By: SteamyTea

Posted By: djhFirstly, energy isn't a 'thing' that you can have 1 kWh of

You can express potential energy in kWh

You can say that some system has a potential energy of X (suitable units) (referred to some transition between states of the system).

There's no such thing as 'potential energy' that exists in its own right of which you can have X (suitable units).

Posted By: djhThere's no such thing as 'potential energy' that exists in its own right of which you can have X (suitable units).

Are you saying that you cannot have a reservoir of water storing potential energy?

SI units are there for a reason and 4 are mainly used in energy calculations

Distance= Meter (m)
Mass= Kilogram (kg)
Time= Second (s)
Temperature= Kelvin (K)

From these we derive the rest
So velocity is ms^-1
Acceleration is ms^-2
Newton (N) is kg.ms^-2
Joule (J) is kg.m^2.s^-2
Watt (W) is J.s^-1

Natural Gas is 53.6 MJ.kg^-1
Gasoline is 46.4 MJ.kg^-1
Diesel is 46.2 MJ.kg^-1
Coal is 24 MJ.kg^-1
Wood is 16.2 MJ.kg^-1
Lithium Battery is 1.3 MJ.kg^-1

Isn't fossil fuels brilliant.

<blockquote><cite>Posted By: Gavin_A</cite>electricity is treated as high grade energy because it can easily be used to do many things including heating with minimal losses, whereas low grade heat can usually only be used for heat, and can only be directly translated to electricity at higher grades of heat in specific circumstances with approx 60-70% losses. Hence them not being viewed as being the same thing.</blockquote>

I agree, but that's only really because we have created a distribution network and devised machines to take advantage of it.

You can equally well meet the power needs of many domestic appliances without using electricity, and even make some usability and reliability improvements too. All it takes is a bit of lateral thinking.

For many homes, for example, refrigeration will be a significant annual cost (energy cost of around £35 to £40 a year plus depreciation cost of maybe a similar amount, assuming a ten year life and around £400 purchase cost). Building in a super-insulated, solar powered absorption cold room would probably have a capital cost of around £500 or so for a new build, but would have a life of many tens of years (there are still 50+ year old absorption refrigerators running today) and would have zero running costs through life. The annual cost would fall from around £80 per year to maybe £20 a year, with an additional environmental benefit of reducing electricity demand and the need to dispose of vast numbers of dead refrigerators each year.

I often wonder what would have happened had we chosen to use pneumatic power instead of electricity. Sure it isn't efficient if you have long distribution runs, but it's relatively safe (using it in the wet being a good example), can be produced from solar heat and can be stored reasonably easily, perhaps in underground pressure tanks for safety. Just a thought experiment really, brought about by thoughts of steam punk.

Never mind all the equations, I have solar thermal panels, and generally speaking they work very well. With UK weather , sunny spells, most days in the summer I get 300lts of hot water from 3 panels - which has frankly exceeded my expectations. You do need to think about when you are going to use your water. The problem being, my ssystem will only heat up if the water in the tanks is 5c less than the temp on the panel. So if you have heated up your tank first thing, then have a sunny morning (having not used your hot water), you have nothing to heat up. PV does not suffer this problem, as they work all the time. Also less maintenace with PV, no liquid, pumps, joints etc etc.

If there has been some sunshine (4 hours or so) we get baths for the kids (shallow) one for us and 2 x showers in the morning. We then hope for sun the next day. They do heat up the water very quickly.

So if going for a lot of panels, you really need to think about a thermal store I guess, designed so you can make the panels work for you, and not spend half the day stagnating.

I am very pleased with mine, but I might have done things differently now I know how they work. Main things is think about how much hot water you use and and what times of the day, and work from there.

RR

Posted By: JSHarrisI often wonder what would have happened had we chosen to use pneumatic power instead of electricity. Sure it isn't efficient if you have long distribution runs, but it's relatively safe (using it in the wet being a good example), can be produced from solar heat and can be stored reasonably easily, perhaps in underground pressure tanks for safety. Just a thought experiment really, brought about by thoughts of steam punk.

Would need your reservoir checked annually for safety and though it is true you cannot get an electric shock from it, it is inherently dangerous and it does not fail predictably (or gently). Also a lot of heat is generated when compressing air, then it cools this leads to condensate that causes problems with lubrication with rotating and reciprocating machinery.
Leaks are also a problem, as is the pipework installation (copper cable carries a lot of current for its size).

All in all it is not a good idea. Noisy as well, would never hear Rabbi Lionel Blue over the whistling of a pneumatic generator.

Edit (10:30pm)
While typing this I had a power cut for about 2 minutes, forgot what they are like.

Posted By: JSHarris1kWh is exactly 360,000J, which can be potential energy in any form you choose. Whether it's heat or electricity makes not a jot of difference, except in as much as some forms of potential energy are easier to use than others, for our domestic purposes.

Personally I would far rather we found better ways of using low grade heat to do useful work in the domestic environment. Low grade solar heat is abundant and fairly easy to collect and move short distances with high efficiency, plus the capital cost of doing so is relatively low when compared with PV.

I'm sorry but both you and ST are still missing my point.

Energy is not a thing, its a measure of a property of a thing. You can't have '1 kWh *OF* energy' in any form. You can only say that the energy *OF* something is 1 kWh.

Posted By: SteamyTeaAre you saying that you cannot have a reservoir of water storing potential energy?

No, I'm saying that the reservoir of water is a reservoir of water. If the reservoir of water is situated some height above a sink for water, then that system has so much potential energy. The energy is a property of the system, not something that exists in its own right. The value of the potential energy of the system varies as the moon and earth co-orbit the sun, for example.

Heat energy is the kinetic energy of atoms (or alternatively, its the increase in their mass due to their motion). Electrical energy is a separation of charges. They are physically different phenomena.

Heat energy provides an excellent example that all '1 kWh of energy' are not the same. Twice the mass of water at half the temperature difference can't produce the same useful work from a heat engine, despite both masses of water containing the same energy. (that's why we also have the term exergy).

Posted By: JSHarrisI've already mentioned using solar powered absorption cooling for refrigeration in another thread, but it bears mentioning again.

Actually, absorption chillers work a whole lot better with a bigger temperature difference. Check out the solar decathlon link I posted, which has good detail of an operational solar-powered unit that I believe is likely to go into production. There are also people building ice houses again (i.e. large rooms to store 'coolth' interseasonally)

Posted By: JSHarrisI often wonder what would have happened had we chosen to use pneumatic power instead of electricity.

Well, we did of course, before we had electricity http://en.wikipedia.org/wiki/Atmospheric_railway and its close cousin http://en.wikipedia.org/wiki/London_Hydraulic_Power_Company but both have been largely replaced by electric power. There are still specialized applications for pnuematic, vacuum and hydraulic power, of course, and undoubtedly there will be new ones arise.

Hi,

On the subject of pneumatics – as an aside waste through leakage is a bigger issue as it is for the water companies, it’s a very key area in reducing operating costs within factories.

As to the storage question it has already been done with the salt dome caverns in a similar way that gas is injected down into caverns ready to be brought back up when required. This is the same principal – Compressed Air Energy Storage (CAES).

With the air storage the air when brought back and released does not drive the turbine as such to generate electricity, but is injected at pressure to remove/negate the need for some of the energy used by the compressor stage of the turbine. So fuel + the pressured air is injected straight into the combustion stage. So it’s a bit like a supercharger except that in this case the air compression is from underground rather than from the engine drive.

See CAES Huntorf Germany

Cheers

Mike up North

Hi,
In terms of energy storage there is a lot being done from pumped hydro, fly wheels, batteries, massive capacitors each have their merits in terms of cost and practical application and very different power delivery profiles. It dosnt get away from the point that the energy held in them has to be generated some how by some thing at the expense of some other form of energy.

Cheers
Mike up north

Fly-wheel storage is even better understood and used a lot. Can be scaled to the domestic size easily and can very easily store 40 kWh.
But back to pneumatics.
How about a large, self expanding bladder attached to the low water line down here, 6 to 9 meter tides twice a day can compress a fair amount of air that can be released through a turbine. For those really stormy days, pipe it to a Wells turbine and take advantage of the 5 meter waves at both low and high tide.
But slightly more seriously, is it not hard to scale up a reservoir as the surface area to volume ratio:
Cube V=6/area
Sphere V=3/area

Basically means that the vessel walls soon becomes very thick.

VH: lots of people do that already (eg see fieldlines.com) with a consumption ~0.1kWh/d, ie maybe 1/10th of a typical stand-up fridge.

However, its inconvenience in terms of access and spills/cleaning and wasted space around it that prevents that layout becoming more common I suspect.

Rgds

Damon

<blockquote><cite>Posted By: renewablejohn</cite>So what is not practical about a steam car. Current uniflow steam engines are in excess of 30% efficient which competes very well with current diesel engines. Steam on demand has been solved by thermal oil steam evaporators. Recharge of fireless boiler can be by Thermal Solar or electric or any fossil or biomass fuel. Extended run times achieved by caustic soda engines. Simples.</blockquote>

Although you can in theory get around 30% thermal efficiency from a steam engine (which still falls a way behind diesel, or even a good petrol engine, like the one in my Prius), it's hard to get this efficiency when using the other big advantage of a steam engine, near-constant torque with rpm. This latter characteristic negates the need for a gearbox, which is a significant advantage.

My friend uses a home made tube flash boiler, which has a low enough steam volume to not fall foul of pressure vessel regs, I believe. It's reasonably practical, but still has a warm up time of around ten minutes or so. I agree, it could be coupled to a solar heating system to reduce this and have near-instant start. Somehow I can't see it catching on as a replacement for conventional propulsions systems though, be they internal combustion or electric.

The exception might be if someone can make a small, safe and affordable nuclear heat source. There would be big PR problems, and safety issues in terms of crashworthiness (as if there aren't those same problems with fuel tanks and batteries), but a closed cycle steam plant like this would make a lot of sense (in my dreams.........).

Posted By: roserambler Never mind all the equations, I have solar thermal panels, and generally speaking they work very well. .................................................................I am very pleased with mine, but I might have done things differently now I know how they work. Main things is think about how much hot water you use and and what times of the day, and work from there.

Can you expand Tim, I'm about to install a 20 x 58mm evacuated tube array, (not yet fixed), together with a simple single coil unvented cylinder, I thought about 180l. This to supply two of us with a summer only DHW supply and to act as a pre heat for my PHx in the winter months.

Olwman,

The dimplex Thermal panels I have seem very efficent. The main reason I went for them over PV's was:
1) we have a HW140 air source heat pump - all installed as part of a major refurb / rebuild out in the sticks, very quiet and didn't like the idea of being in the garden oin the summer and having it come on.
2) we only had a small south facing roof - and thought I would have less impact with PV witht eh space.

The mian things I have found - is the panels do a great job at heating the water. Most sunny days we get a full tank (300L) of hot water. The probelm is - planning your use so you maximise the suns rays. The water in the tank will only be heated if the temp in the tank is 5c lower than the temp on the panels. So, if you think, many days the temp on the panel doesn't get over average of 35c.. It is not unusual for the tank temp after evening and morning use to settle around the low 30's. So much of the day, the sun's energy is not being utilised. This also happens in very sunny spells too. In april this year, the tank had reached max temp of 60c usually by early afternoon. So unless, you have somewhere to route the hot water - the panels simply stagnate. Not a problem, but undermine the overal efficency of the set up.

So my point is, you need to think carefully about how you will use them to get the most out of them i.e. maximise day time hot water useage (difficult for most people - particularly as dishwashers and washing machine don't use HW).

Providing the two of you are prepared to put a little planning into the water useage of the day, and time the top up heating of the DHW to work around your needs and the sun's rays - it should be good.

I have only had the system since march, so I don't know how the pre-heat thing will work in the winter. The problem as ever - is balancing the conflict of wanting a tank of hot water against, have the temp low enough in the tank to use the energy from the sun - generally when temps aren't high in the winter.

Whisper me if you need anything more.

RR

Mike,

It does sound like your system would avoid the issues I have highlighted. I am no expert, and some of the people commenting in the above debate might now be able to help you with a little more science, but working from experience - the key with ST is having somewhere for the hot water to go, and making sure it doesn't spend to long stagnating.

If you can overcome that, I would advise to fill your boots. The last few days have been sunny here and may modest array is providing 300L of 60c water easily. If your have a south(ish) orientated roof, its probably worth the investment. Grants and tariffs are all well and good, but do you really trust any future government not to change what is in place? - I don't. Go for the most efficient system, that suits your needs.

Tim