What is the most efficient method of heating large volumes of water?

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bcraig

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#172065 10-May-2015 17:53

I'm hoping to get some advice on a the most efficient way to heat large volumes of water.

I thinking of running 6 built in 1300-1500 Litre tiled, insulated spa baths which will all possibly join onto one water management system. After each use I want the spa to drain completely for a quick wipe down and maintain maximum cleanliness. I am thinking the water will be reused up to a point depending on the amount of use it has had.

One idea is that the water that fills the bath could be heated with a high volume electric or gas instant water heater. I like his idea because this allows the bath to be set to a preferred precise temperature for filling. The water would then be drained, filtered, and treated then stored in a insulated tank until next required. Though the stored water will lose heat over time, if the baths are refilled regularly I see that the water will retain some of its original heat and will need less energy to reheat.

The second idea is to have the water boiled in a electric or gas tank heater. There could be one tank for each bath or maybe one or two large tanks to boil water for all baths. These tanks would be running 24/7 and each bath could fill from these with another small electric or gas heater on the way to achieve the precise preferred temperature. The drained water would again be filtered, treated and transferred back into the heater tanks where it would need to be slightly reheated and maintained again.
Once the water is approaching its maximum usage and needs to be dumped and replaced then using the heater tank method would draw a lot of energy to bring this water back up to temperature. Lets assume this will be up to once or tice a week.

So which do you think would be the most efficient on this scale in the long run? Electric/gas hot water on demand or electric/gas stored hot water?

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#1301532 10-May-2015 17:55

Solar. Heat pump hot water heating.

Do you want time efficient, cheap, or other?

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gzt

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#1301537 10-May-2015 18:02

Do they have built in heating units for maintaining temperature?

bcraig

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#1301541 10-May-2015 18:18

@timmay - I would prefer to avoid solar at the moment but I could look at that down the line. Speed and efficiency is key. I will need a fast bath refill and potentially a high turnover of refills and doing this the most reliable and cheapest way.

@gzt - Not as such. They will only have the option to activate jets which will pump out recycled water with heated air injected into it. The water will only be used in each tub for up to an hour session before being drained again. I'm assuming the water will not loose too much heat to be noticeable. Otherwise I guess I will need to look into heaters to maintain the temp throughout the hour as well.

richms

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#1301555 10-May-2015 18:45

Im trying to think what sort of place would need this and my mind just wont leave the gutter.

In anycase, an hour and the water will be cold without a heater in a spa bath.

Richard rich.ms

Niel

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#1301556 10-May-2015 18:46

Gas will give you speed.

Heat pump (works the same as an aircon unit) will give you highest efficiency, and will be 3x faster than an element consuming the same power. Better still, you get large heat pumps for swimming pools which will probably be your best option if this is for business use (i.e. you can spend the cash). There are some on TradeMe.

You could even combine gas and heat pump, and use the one that is required at a particular time.

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bcraig

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#1301560 10-May-2015 19:00

richms: Im trying to think what sort of place would need this and my mind just wont leave the gutter.

In anycase, an hour and the water will be cold without a heater in a spa bath.

Lol! A private, family friendly spa pool service. Ok I will defiantly look into maintaining the temperature with further heating.

Neil: Gas will give you speed.

Heat pump (works the same as an aircon unit) will give you highest efficiency, and will be 3x faster than an element consuming the same power. Better still, you get large heat pumps for swimming pools which will probably be your best option if this is for business use (i.e. you can spend the cash). There are some on TradeMe.

You could even combine gas and heat pump, and use the one that is required at a particular time.

Cool I will look into heat pump units. That could a good idea combining different types of heaters depending on the requirement at the time. Possibly have gas as a backup for high demand.

richms

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#1301564 10-May-2015 19:11

In large houses, solar is often used as a pre-heat for a gas system. At least some of the continous ones will let you use almost hot water into them and they will take it the rest of the way.

Also there are some systems around that are a heat exchanger in your waste water which will preheat the water going into a hot water system, that would stop as much heat going down the drain each time.

Richard rich.ms

raytaylor

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#1301647 10-May-2015 23:22

I would think that a pure electric califont would be best - thats basically an element boiling water like a kitchen kettle.
Gas is generally faster when cooking.

There are heat pump hot water systems which are more power efficient - probably not as fast.

When buying a kitchen stove, gas is the faster way to boil a pot of water because the element / contact surface heats up faster.
Electricity is considered slower, because the element uses less kilowatts than a gas burner, per hour. So gas with a larger burner, can deliver more kilowatts of energy in the same time as a typical electric element. However if you use a larger element, that consumes/transfers more energy then it could be just as fast - but the retail market doesnt offer that in a typical residential product. Commercially, it might be different.

Now I dont know much about gas costs but I'll try and give you some cost ideas for electricity options.

The equasion to heat 100 litres of water by 1 degree over 1 hour is
100L x 4 x 1 degree / 3412 = 0.1172 kWh

To heat 1400 litres of water from tap temperature (15c) to spa temp (38c) with 23 degrees difference:
1400L x 4 x 23 degrees / 3412 = 37.74 kWh

Now i dont know what kind of plant your local lines company has in your area, but that is a f*load of power.
You could take say 5 hours to heat the water - that could be done with a 7.5kW supply.

1) Direct electrical element
Not as fast as gas in general - though if you can get a bigger element, you can use more power to make it fast.
No matter how long it took to heat the water (your power supply / element size determines this) it will cost you $8.68 to heat the water at 23 cents per unit / kWh

2) Heat Pump
Most heat pump systems are 3 to 4x efficient. That means they can extract heat out of the air and transfer it to the water and use only 1 kilowatt of electricity for every 3 kilowatts of heat transfered from one side (outdoor unit) to the other (water)
Rather than using electricity to heat the water directly, they just use electricity to move heat from one side to the other.
It will therefore cost you $2.89 to heat the water.

I dont know much about the cost of gas, but i would think that a heat pump is still cheaper - though it may not be as fast.
Your idea of using an insulated tank is a good one.
If you can pre-heat the water using a heat pump system, then use a gas califont to achieve the desired temperature as you say, it would be doable.

Your situation is also what makes solar very efficient.
You can use electric solar panels to run a heat pump system - with any solar installation, the more power you consume during the day, the faster they pay off.
When you store a larger volume of water, it will take longer to cool down as the surface ratio of the tank is lower per litre stored. So there is a benefit to over-sizing your storage tank and heating extra water during the day so you dont need to use grid power at night. A smaller tank would need more heating as there is more loss of heat per litre of water between sunset at 5pm and spa time at 9pm.

Two or three heat pump units could run at the same time - a typical home hot water tank has a 3kW element in it.
3000 watts going through some heat pumps is equivalent to 9000 kW of heating capacity.
That means you could heat 1400L to 38 degrees from tap temp in 4 hours 12 minutes.

I am guessing that you are running a public facility? I am not quite sure how big 1400L of water is in relation to the room I am sitting in now.
The typical rinnai gas system we have in our home has a temperature setting on the wall panel in the kitchen and bathroom. Water goes in, and it heats it to the desired output temperature. This means when we get in the shower, we only use hot water (set on the rinnai panel) and dont mix it with cold on the shower tap. The rinnai califont therefore only needs to heat the water to the desired temp and no hotter, only to be mixed with cold water again at the shower head.

Now if you are worried about water re-use, I has another idea.
If you could flush the used water into a second tank, and use this as your outdoor unit / heat source, that means the heat pump system can become super-super efficient at moving the heat across because it doesnt have to work so much to extract heat out of the air or source - they are most efficient when the source and destination temperatures are the same and it only has to increase one side by a few degrees.
So you could re-use some of the water, and extract the heat out of the remaining water?

Ray Taylor

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bcraig

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#1301702 11-May-2015 08:47

richms: In large houses, solar is often used as a pre-heat for a gas system. At least some of the continous ones will let you use almost hot water into them and they will take it the rest of the way.

Also there are some systems around that are a heat exchanger in your waste water which will preheat the water going into a hot water system, that would stop as much heat going down the drain each time.

From the advice I have been getting it seems that I would be a fool not to consider solar straight off the bat. My concern was having enough panels that would fit on a commercial lease building that would actually come to a benefit. I am starting to see ways of not using as much energy as I thought I would need and now solar if definitely looking more sufficient.

I love the idea of a heat exchange. Hadn't thought of that. At peak I could be dumping 10,000L of water a week and to be able to reuse that energy would be great. I wonder if that amount of water flow would generate significant energy on a small hydro-turbine to further maximize its use.

raytaylor: I would think that a pure electric califont would be best - thats basically an element boiling water like a kitchen kettle.
Gas is generally faster when cooking.

There are heat pump hot water systems which are more power efficient - probably not as fast....

Your equations have opened my eyes to a better outlook on energy consumption. Where did you source the equation and can you explain the constants? (x4, /3412). Do you have an equation for calculating heat loss in a mass of water? I would like to calculate how much energy would be needed to maintain the temperature in a tank.
I have found a heat pump pool heater that takes 12kw in and puts 60kw out. So it should be able to heat 10,000L to 38 degrees in under 5 hours. Then I can use a smaller instant water heater to make up for a higher preferred temp if needed. This could be used on each bath with a recycling system to maintain the bath temp during its use as well. Hmm then again should I look at another small heat pump heater instead for this purpose?

Your idea of a second waste tank as a heat source is great. I could see it being an overflow tank so my weekly dumps from the baths would be moved to the overflow tank which will always maintain a reasonably warm state and it will self empty/overflow to the waste/sewer.

trig42

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#1301707 11-May-2015 08:55

It is going to be expensive to heat, which is why you generally only see these places in thermal areas (Have you been to the Te Aroha hot pools? - they empty their tubs after every use, but the hot water comes out of the ground).

What are you going to use to filter the water to re-use it? After sitting in the pools at Te Aroha for just half an hour, the amount of oils and dead skin you see floating on the surface is pretty gross (hence why they empty them). Also, half an hour at Te Aroha is $18pp, which I consider well up there in price - their hot water will be a lot cheaper than yours - how much are you planning to charge? (http://www.tearohamineralspas.co.nz/index.php/prices)

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#1301712 11-May-2015 09:02

Is the pool heat pump capable of putting out 38 degrees? You want to get the input to the gas califont as hot as possible before actually consuming gas. So if the desired temperature is between 30 and 40 degrees, you at least want to bring the input up to 30 degrees so gas can heat the rest. The heating you do with a heat pump is going to be cheaper than the heating with the gas.

I would have thought a pool heat pump would be designed to have a lower heat output but be able to pump much more water through it in litres per hour to keep it more efficient
I base that idea on the fact that a heat pump is more efficient when both sides are equal and it only needs to raise one by a few degrees - the sooner the warmer pool water is pumped out, and replaced again by equal temp water to be heated means it is not going to have to try harder to heat.

I could be wrong on that though

The equasions are just a standard that I learned in 3rd form science class.
I think its because the actual numbers are in BTU (British thermal units: 1deg x 1 kg = 1 BTU to heat?) and to convert from kilowatts via BTU the constants are required.

Heat loss from a mass of water - it is going to be hard to work this out.
I cant remember the equations, but if you are insulating the tank, pipe and baths then it makes it harder to work out.
The trick will be to over-insulate but there is a point where insulation costs more than the heat it is trying to keep in.

My initial thoughts would be to take a farm tank from farmlands or RD1, and cover it with bands / strips of foil - shiny side in. Then get some of those underfloor insulation polystyrene things and glue them over the the tank - with two or three layers. Then wrap the whole thing with another layer of foil. Polystyrene is what they use in a chili bin - and that can keep a block of ice cold and keep it below 1 degree for a number of days. The underfloor stuff is similar to polystyrene - not as good, but it is more fire proof so hence the multiple layers.
A chilibin can keep a block of ice below 2 degrees for a number of days - or a temperature rise of maybe 10 degrees per 50 litres over 3 days.
Here is a good one - http://www.trademe.co.nz/sports/fishing/other/auction-884516831.htm 5 days outdoors and its still under 3 degrees C / 38 degrees F

With regards to solar - a typical home can fit a 3.6kw system on the roof.
My home can fit a 8.4kw array as I have flat angled roof of 14m x 4m which unfortunately doesnt face north.

Ray Taylor

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k1wi

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#1301719 11-May-2015 09:08

Heat recovery from your waste water is going to be key to keeping your costs low. Both in terms of not flushing heat down the drain and in increasing the capacity of whatever system you use.

blackjack17

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#1301723 11-May-2015 09:21

bcraig:
richms: In large houses, solar is often used as a pre-heat for a gas system. At least some of the continous ones will let you use almost hot water into them and they will take it the rest of the way.

Also there are some systems around that are a heat exchanger in your waste water which will preheat the water going into a hot water system, that would stop as much heat going down the drain each time.

From the advice I have been getting it seems that I would be a fool not to consider solar straight off the bat. My concern was having enough panels that would fit on a commercial lease building that would actually come to a benefit. I am starting to see ways of not using as much energy as I thought I would need and now solar if definitely looking more sufficient.

I love the idea of a heat exchange. Hadn't thought of that. At peak I could be dumping 10,000L of water a week and to be able to reuse that energy would be great. I wonder if that amount of water flow would generate significant energy on a small hydro-turbine to further maximize its use.

raytaylor: I would think that a pure electric califont would be best - thats basically an element boiling water like a kitchen kettle.
Gas is generally faster when cooking.

There are heat pump hot water systems which are more power efficient - probably not as fast....

Your equations have opened my eyes to a better outlook on energy consumption. Where did you source the equation and can you explain the constants? (x4, /3412). Do you have an equation for calculating heat loss in a mass of water? I would like to calculate how much energy would be needed to maintain the temperature in a tank.
I have found a heat pump pool heater that takes 12kw in and puts 60kw out. So it should be able to heat 10,000L to 38 degrees in under 5 hours. Then I can use a smaller instant water heater to make up for a higher preferred temp if needed. This could be used on each bath with a recycling system to maintain the bath temp during its use as well. Hmm then again should I look at another small heat pump heater instead for this purpose?

Your idea of a second waste tank as a heat source is great. I could see it being an overflow tank so my weekly dumps from the baths would be moved to the overflow tank which will always maintain a reasonably warm state and it will self empty/overflow to the waste/sewer.

I am not sure the equations are right?

quantity of water * specific heat capacity * temp change / time (s) = kW/h

It takes 4.2 joules of heat energy to raise the temperature of 1 mL of water by 1° C
therefore 4.2 kJ of heat energy to raise 1L of water by 1° C

so to be more accurate the 4 really should be 4.2

and I assume the 3412 is seconds in an hour so shouldn't that be 3600 (60*60)

which would mean the correct equation would be

1400L x 4.2 x 23 degrees / 3600 = 37.56 kWh (almost the same i know)

You would also have to account for wastage (10% a cycle?)

1400L = 14m3 and if you had a used tank and a fresh tank that two of them, they would take up a lot of space

(I'm a science teacher but not a physicist)

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trig42

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#1301727 11-May-2015 09:29

blackjack17:
bcraig:
richms: In large houses, solar is often used as a pre-heat for a gas system. At least some of the continous ones will let you use almost hot water into them and they will take it the rest of the way.

Also there are some systems around that are a heat exchanger in your waste water which will preheat the water going into a hot water system, that would stop as much heat going down the drain each time.

From the advice I have been getting it seems that I would be a fool not to consider solar straight off the bat. My concern was having enough panels that would fit on a commercial lease building that would actually come to a benefit. I am starting to see ways of not using as much energy as I thought I would need and now solar if definitely looking more sufficient.

I love the idea of a heat exchange. Hadn't thought of that. At peak I could be dumping 10,000L of water a week and to be able to reuse that energy would be great. I wonder if that amount of water flow would generate significant energy on a small hydro-turbine to further maximize its use.

raytaylor: I would think that a pure electric califont would be best - thats basically an element boiling water like a kitchen kettle.
Gas is generally faster when cooking.

There are heat pump hot water systems which are more power efficient - probably not as fast....

Your equations have opened my eyes to a better outlook on energy consumption. Where did you source the equation and can you explain the constants? (x4, /3412). Do you have an equation for calculating heat loss in a mass of water? I would like to calculate how much energy would be needed to maintain the temperature in a tank.
I have found a heat pump pool heater that takes 12kw in and puts 60kw out. So it should be able to heat 10,000L to 38 degrees in under 5 hours. Then I can use a smaller instant water heater to make up for a higher preferred temp if needed. This could be used on each bath with a recycling system to maintain the bath temp during its use as well. Hmm then again should I look at another small heat pump heater instead for this purpose?

Your idea of a second waste tank as a heat source is great. I could see it being an overflow tank so my weekly dumps from the baths would be moved to the overflow tank which will always maintain a reasonably warm state and it will self empty/overflow to the waste/sewer.

I am not sure the equations are right?

quantity of water * specific heat capacity * temp change / time (s) = kW/h

It takes 4.2 joules of heat energy to raise the temperature of 1 mL of water by 1° C
therefore 4.2 kJ of heat energy to raise 1L of water by 1° C

so to be more accurate the 4 really should be 4.2

and I assume the 3412 is seconds in an hour so shouldn't that be 3600 (60*60)

which would mean the correct equation would be

1400L x 4.2 x 23 degrees / 3600 = 37.56 kWh (almost the same i know)

You would also have to account for wastage (10% a cycle?)

1400L = 14m3 and if you had a used tank and a fresh tank that two of them, they would take up a lot of space

(I'm a science teacher but not a physicist)

1400L = 1.4 m3 - 1 Cubic Meter = 1000 Litres = 1000 kg (assuming pure water 1ml = 1g).

mclean

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#1301773 11-May-2015 10:22

You probably need to think about how you're going to manage the pools. For example, do you want individual temperature control at each pool? Are you going to use a central filtration plant or one for each pool? Do you want to have the pools ready and up to temperature when customers arrive?

If you use a central filter you can't have individual temperature control. If you want pools to be available for customers when they arrive then you need to store the water in the pools, not in a holding tanks. Which means a small filter and heater at each pool. This will also give you better availability because if something breaks down it's unlikely to knock out the whole lot.

Apart from when you backwash the filters the only heating demand is the standing loss from the pool. This is loss into the pool shell plus evaporative losses from the surface. You need to get the best insulation you can into the pool walls, and always cover it when it's not in use. There's a lot of evaporative loss from spa pools because of the high water temperature, especially if you ventilate the space. If you take care of these then a standard electric spa pool heater at each pool might be the way to go.

With this arrangement you would only drain a pool down to your holding tank while to clean it, then immediately return it to the pool and get it back to temperature.

The next thing to look at would be backwash heat recovery. That won't be cheap but you could add later once your cashflow works out. Some places actually clean and re-use the backwash water, but backwash water can be pretty horrible so you have to be very careful doing this.

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