How much energy to heat 100 liters of water?

[bernard]

Originally Posted by Daox

1kw of electricity equals 1kw of heat. Its a direct relationship.

Thank you Daox.

So it's 1kw of electric energy equal to 1kW of thermal energy?
"heat" is thermal energy?

[Daox]

Yes, in my example, heat and thermal energy are the same.

[bernard]

Thank you Daox,

So 4 kWh in my example, is in fact a thermal energy, not an electric energy:

a) electricity:
volume * (deliveryT-mainsT) / EF
100 * (45-10) / 1.0 = 3500 kcal
3500kcal / 860 ≈ 4 kWh of thermal energy?

And what is 5kWh in my example:

b) natural gas
volume * (deliveryT-mainsT) / EF
100 * (45-10) / 0.8 = 4375 kcal
4375kcal / 860 ≈ 5 kWh of ?

?

[jeff5may]

"DirecTV, I already told you."
- john cleese

[Michael34]

Volume in liters x 4 x temperature rise in degrees centigrade / 3412

For Example 100 liters of water, to be heated from 20ºC to 50ºC, giving a temperature rise of 30ºC would give –

100 x 4 x 30 / 3412 = 3.52(meaning that the water would be heated in 1 hour by 3.5kW of applied heat.)

[gtojohn]

in imperial terms is takes 1 btu to raise 1 pound (pint) of water1 degree. resistance heat like electric hot water produces 4.13 btu per watt. not very efficient in terms heat per watt. if you are producing your heat with a heat pump you might expect to produce 12 btu or higher per watt of electricity input.

[jeff5may]

Originally Posted by gtojohn

in imperial terms is takes 1 btu to raise 1 pound (pint) of water1 degree. resistance heat like electric hot water produces 4.13 btu per watt. not very efficient in terms heat per watt. if you are producing your heat with a heat pump you might expect to produce 12 btu or higher per watt of electricity input.

Nope, the few previous posts state the correct conversion rate of 3.412 btu per watt. However, many gas heated units are Inefficient enough that they run at or above this rate. But electrical heated units generally stay below 3 1/2.

The 4.13 value is very close to the conversion rate of btu to kilocalories. Kind of like rounding the value of pi off to 3.14.

[gtojohn]

Ok, you caught my bad math, 3.412 is as efficient as resistance heat can be. But my main point was you could produce more hot water with the same amount of power with a different method of heating. 4kw resistance heat vs 4kw heat pump. Since the original post was rather open ended,"how much energy to heat 100 liters of water?" I propose using 1/4 as much with a heat pump.

[stevehull]

Go to

You are exactly correct that a heat pump would heat water with 1/4 the energy as compared to a straight resistance type electric heater. The 1/4 number, when expressed as the reciprocal is called the coefficient of performance (COP) and in this case the COP is 4.0

I am designing the plumbing water heating infrastructure for our new house and you can purchase water to water heat pumps with a COP of 5+ with inlet water at 60 F (central Oklahoma) and an output temp 50 F degrees higher (110 F).

This is also about 1/2 the cost of natural gas and 1/4 the cost of propane. A good storage water tank is in the range of $500 and the annual water heating using electricity would be ~ $400. Using the GTHP with a COP of 5 (cost ~ $2) means an annual water heat cost of ~ $80 - or some $320/year in savings or ~ a five year payoff.

And I believe I can make one for a LOT less than $2K!


Steve

[Fornax]

Originally Posted by bernard

But 5 kWh of what? Not an electric energy, but what? Thermal energy?
How can I compare 4 kWh of electric energy with 5 kWh of Thermal energy?

There are several types of energy but the quantity is always measurable in joules (J). 1 joule of electrical energy is exactly as good as 1 joule of chemical or potential or kenetic energy.

Also one type of energy can be converted into the other, the amount of energy involved always stays exactly the same, you just don't always get 100% of what you want. An electrical heatingelement converts electricity in heat, and does that almost 100% efficient. Your car-engine uses the chemical energy present in petrol to bring your car in motion, but it's not very efficient, a lot of energy is 'wasted' in the form of heat. (But the total amount of energy stays exactly the same.)

The trick is to get the form of energy you want by using as little energy in total as possible. This is better for the environment and often better for your wallet, although sometimes that takes a few years.
For example, an electricityplant (gas/coal) converts chemical energy into electricity, but it's roughly 35% eficient. A gasfurnace for the central heating of a house is roughly 98% efficient nowadays. So heating your house with natural gas is more efficient than using electrical heating.
But if you look at a heatpump with a SCOP higher than 3 (And they exist with a SCOP of higher than 5) then it is suddenly more economical to use this inefficiently produced electricity for heating your home.

But a short answer to your question: they are all measured in joules, or kWh or BTU or kCal because the amount is the same, it's just another form of energy.