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Geysers in Series


DeepBass9
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I have an old 300 l flat plate collector solar geyser which will eventually expire (I can see it coming). When I replace it I am thinking of getting 2 150 l EV geysers an putting them in series so that if I need to use Genny power to heat water in winter, I can select whether to hear 150 l which will be quicker , or 300l which will be slower. Are there any pit falls in doing this?

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It's pure resistive elements, so electrically it will work.

It will take longer to heat the water of course. You're doubling the resistance and therefore halving the current, so it will be a 1500W combined element.

You have twice the amount of water and half the power, so it will take 4 times as long to heat the water.

It takes 1.16wh to heat one liter of water by one degree centigrade. Assuming ambient of 15 Celsius, and a target of 55, you have a delta of 40. 1.16*40*150 ~= 7kwh (per 150l tank). Water heats linearly, so a 3kw element would normally take 7/3 = 2.33 hours to do the job (per tank), so the combined setup will take 4 times that, or 9 hours and 20 minutes from cold.

Standing loss on each tank is around 2kwh per day, or 4kwh combined. 4/1.5 = 2 hours 40 minutes eacȟ day just to replace the lost heat (no consumption).

So its going to depend on how long your generator will run. Seems to me that at best you'll counter some of the standing loss when running in series.

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I installed a 200l solar geyser wanting to add a 2nd one later. Was told afterwards it would have been better to have gone for 400l from the start.

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55 minutes ago, plonkster said:

It's pure resistive elements, so electrically it will work.

It will take longer to heat the water of course. You're doubling the resistance and therefore halving the current, so it will be a 1500W combined element.

You have twice the amount of water and half the power, so it will take 4 times as long to heat the water.

It takes 1.16wh to heat one liter of water by one degree centigrade. Assuming ambient of 15 Celsius, and a target of 55, you have a delta of 40. 1.16*40*150 ~= 7kwh (per 150l tank). Water heats linearly, so a 3kw element would normally take 7/3 = 2.33 hours to do the job (per tank), so the combined setup will take 4 times that, or 9 hours and 20 minutes from cold.

Standing loss on each tank is around 2kwh per day, or 4kwh combined. 4/1.5 = 2 hours 40 minutes eacȟ day just to replace the lost heat (no consumption).

So its going to depend on how long your generator will run. Seems to me that at best you'll counter some of the standing loss when running in series.

Hell thanks @plonkster now this is a lekker piece of info that I am saving somewhere!

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My brother installed 2 x 200l Geyser in Series. Both have their own 20 EV Tubes connected to it running from 2 Geyserwise. 

Works perfect.  The water entering the second geyser is already warmer so it does heat quicker by EV or element. You also don't need 400lt at 55C, Only 200lt at 55C. Reducing heat loss.

Benefit of 2 geyser is that you can set different temps and can change them through the year. ie. In summer you drop the first geyser's element setting to 30C (i.e Does not come on), in winter to 45C. You can also change it if you are having people stay over and you need 400lt at 55C. Back to 200lt at 55C when they leave.

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Another Benefit: 2 Geyser = 2 Elements

You can change the "cold" geysers element to a 1.5KW element, and link it onto your inverter, and control it with a Relay from your BMV/Charger.  When your BMV gets to 100% or your charger goes to Float it switches on..  Result - You dump your excess electricity generation in your geyser. Still having the second "warm" geyser on Eskom.

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1 hour ago, The Terrible Triplett said:

I installed a 200l solar geyser wanting to add a 2nd one later. Was told afterwards it would have been better to have gone for 400l from the start.

The heat loss will be proportional to the outside area of the cylinder. That is also the optimum amount of material needed to make a container of that size. Since the volume grows as the square of the diameter, you only sacrifice a tiny bit more surface area for double the capacity. But if you go with two geysers... well then you have double the area, double the material, double the standing loss :-)

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But @Louw makes a good point. My calculations are only accurate if you take both tanks to the full temperature. If you don't actually need 300 liters of hot water per day, then change it into a prefeed system (one tank feeding the other) and you can play with different sized elements, timing, control systems, sky's the limit.

Also, should be noted that the standing loss figure of roughly 2kwh a day is measured at 65 centigrade. At a lower temperature it is of course much lower.

The cooling equation is T = ce^kt (c times e to the power of k times t), where c and k are constants and t is the time (in whatever unit you prefer, but usually seconds), and T is the temperature at time t.

To solve for c, plug in t=0. e^0=1, so c will be the starting temperature (at t=0), lets say 55.

To work out k is a bit more complex. What I usually do is use the standing loss to work out the temperature after 24 hours. Using the same 1.16wh per centigrade per liter formula, you can turn the standing loss into a temperature. Plug in that temperature, with c=65 and t=24 hours, and you can work out k using a natural logarithm.

Then you can make a nice plot specific to your own geyser.

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I am using this setup at the moment. One 150 litre geyser is pre heating the water with a geyserwise / EV pump unit. Its element is disabled and it doesn't use electricity. My second geyser (200l) has an element with a controller, with the EV pipes running straight into the geyser. It only turns on if the water temperature drops below 55 degrees and it receives warm water from the 1st geyser.

Benefits:

  • Your main using geyser is the only one with an element. 
  • It doesn't get cold water in, the water it receives is usually 55 degrees or more, which means the primary geyser water remains hot
  • Its element works seldom, so the geyser lasts a long time and it doesn't use much power
  • EV running straight into the geyser is MUCH more efficient that an EV flat panel, pump & geyserwise setup
  • My primary geyser is usually at 75 Degrees plus

Drawbacks

  • It isn't very efficient when it is cloudy. Although my electrical solar still generate about 50% power, this drops to about 25%. If its raining there is no heat and it will go onto grid / your solar power system
  • There is some heat loss in transferring hot water from one geyser to another. Ensure your pipes are well insulated.

If I have to replace the primary geyser or the system, I would do it differently. I would fit a larger 250 - 300 litre geyser with the EV running straight into the geyser. This, when using the sun is much better than the pump / geyserwise setup. Those setups in winter keeps circulating the water at night to prevent your pipes to freeze and really stuffs up your hot water generation. Following this, I will put in a proper gas geyser like the new bosch ones, that measures water temperature at inlet and outlet and only turns on if the water at the inlet is below a certain temperature. Also ensure it has enough flow for your shower & bath to run simultaneous and does not have a pilot flame (Auto ignite). These units are not cheap, last price I got was in the region of about R18k. You also need a gas installation in your house, which most people running solar have because it is much more efficient for cooking.

The benefits with such a setup is that it will only switch to gas when your solar isn't working, thus eliminating the need for an element in the solar geyser.

 

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1 hour ago, plonkster said:

The heat loss will be proportional to the outside area of the cylinder.

FWIW, my geyser is as is mounted outside the house, no cover whatsoever. If you touch it, the metal is cold even when the geyser is on temp.

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2 minutes ago, The Terrible Triplett said:

FWIW, my geyser is as is mounted outside the house, no cover whatsoever. If you touch it, the metal is cold even when the geyser is on temp.

Yeah it's pretty well insulted. Newton's law of cooling still applies though :-)

edit: That is to say, T is still equal to ce^kt, the good insulation just provides a better "k".

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  • 1 month later...

I would like to ad a new angle in the hot water.B)

Out of necessity I installed my old copper geyser 150L against my chimney as high as possible between the ceiling and the roof. Advantage is that with a simple T connector coupled to the inlet and hot water outlet I could use the thermo syphon effect from a flat plate collector salvaged from a scrapyard for R500. Past winter I realized that I had the opportunity to use the heat of my fire to heat my geyser... again two T's on the in and out of the geyser. One copper pipe (hot return) running up inside my chimney passing through the chimney into the geyser. the cold feeding line running down the outside of the chimney entering the firebox from a lower level than the hot water pipe going up the chimney. I have two pieces of about 300mm copper exposed to the heat. With a heat sensor on my geyser I can manipulate the fire not to get the geyser to hot! I can easily raise the temp by 15 to 20 degrees...

The big benefit is measured in the triple bottom-line ie. (a) Financial advantage (b) Social benefit with family and pets enjoying the fire and heated house  (c) ecological benefit despite the burring of wood I am lead to believe that wood fires are less damaging than coal to the environment.

After one year of use I really enjoyed the system.

On the picture two additional pieces of copper was soldered onto the water pie to try and get more heat transferred onto the water pipe... these two pieces burnt away within a month of use... not worthwhile:(

20160519_134540.jpg

20160519_142423.jpg

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Nice to hear that it works! I am busy with a similar project to catch heat from our wood stove to heat water for the kitchen basin. I have a similar copper pipe heat exchanger built into the chimney, I just need to connect it up to a small EV geyser.

Meanwhile my generator cooling water healing system is working a charm. I even have an electric heater again which I use to add to the load on the generator if there is no other load to get the cooling water nice and hot.

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You have inspired me to finish off my project at the weekend..... Currently for washing up we go an fetch a bucket of hot water from the laundry, or boil a kettle on the gas or wood stove. I have the geyser, I just need to do a bit of plumbing.....  

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Why not use a copper (car) radiator higher up in the chimney? If you can size one to the same inside size of the chimney you wil utilize a lot more of the heat going out.....
I was busy with a project like that but with the price of wood and the fact that we don't light the fireplace that often i just went straight to a lpg instant water heater. Easy to plumb, sits outside out of the way, much smaller than electric geyser, no more cold showers because the heat is instant. It's got a lcd screen where you can adjust the temperature and the flame is ignited by 2 torch batteries. At just over R3k it seemed like a much better option at the time. First 48kg lpg lasted just over 6 months (water heater and 5 burner hob) and cost R852 to refill. You do the costing (initial installation as well)

Sent from my S60 using Tapatalk

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5 hours ago, DeepBass9 said:

 

As long as there is water in the pipe it should be OK, when the pipe gets too hot the copper will start to oxidise away (I guess?)

Also soldered joints may melt and push open if there is no water inside.

 

I did this many years ago in my braai , thought I would be smart and have a hot water tap when we make fire. Soldered the whole thing up with t joints and all. It was a closed system so I decided to install a 3 kpa pressure release valve just for incase. LOL was I wrong the heat inside the pypes got so hot that the solder melted even with water in them. (Have you ever tried to solder a pipe with water in it :0 ) .
As I understood it the water got super heated as there was pressure so I ended up installing a tank with a ball valve where the water would boil and get topped up. That seemed to work well enough for the one tap..

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While we are on the geyser elements  subject:

I have a 200l geyser with a 4kW element. I then change the 4kw to a 2kw element to match my solar system.   Axpert 4kw

Because the stupid inverter resets the MPPT every time the element switches my batteries are now "moeg". (40A @ 50Vdc = 2000w)

So i thought ill be smart. I then changed the 2kw element for a 1kw element :-(.    The element needs to be on the whole day other wise the water is to cold for the evenings showers. This is a bit of a challenge now with winter here. 

The next step i want to try is to connect a separate PWM solar charger to the battery bank. I hope this will help the batteries while the MPPT resets. 

Have anyone done this before.   MPPT and PWM connected to the same bank?

 

 

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5 minutes ago, Fritz said:

Have anyone done this before.   MPPT and PWM connected to the same bank?

Yes. Need 2 arrays,

Don't.

The two will argue i.e. I am charging, no I am, no I am until one takes the lead other one steps back.

A Suggestion: Using the Axpert inverter, disconnect the array from it, connect array to a separate MPPT controller direct onto the batts maybe?

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13 hours ago, Czauto said:

You do the costing

I did that, many moons ago, just working off the per-kg cost of LPG, the inherent energy content of LPG (converted to KWH). If I remember this correctly, a 9kg bottle has the equivalent of 115kwh, which you can work out by googling "calorific value of lpg", then converting the 46.1MJ to kwh (google for "convert 46100 joule to wh"), then multiplying that by however many kilos in your bottle, I used 9. The bottles aren't always filled completely to 9kg either, so this value is optimistic. Refilling a 9kg of gas costs around R230 or so, buying that amount of electricity costs between R230 and R260. In other words, per unit of energy gas and electricity is on par. The electric water heater is however almost 100% efficient, and I can guarantee that gas won't be anywhere near that... so electricity wins.

However, now that we have water problems, and given that the electrical heater doesn't win by that much... the instant-hot feature of gas plus the low installation cost, not to mention the independence given that we are threatened again by load shedding, gas is a very very compelling option.

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