SOLAR HOT WATER DIY
This section is about do it yourself solar hot water.
We have many overcast days in NZ I am looking at ways to get useful solar hot water on overcast days. As well as good solar performance it has to be inexpensive. If I can make my own hot water using solar energy at a low price I will be very happy.
I bought two solar units commonly avaliable in New Zealand a thermosyphon unit and a heat pipe unit for testing.
A third type of solar unit sold in New Zealand is the U tube type which have glass tubes and tubes inside which carry water. Because there is water in the interior tubes these types could freeze in Winter so they use a water/glycol antifreeze mixture and use a hot water cylinder with a heat exchanger coil to isolate the glycol mixture from your usual hot water. Because this type of unit is not easy for DIY I am not testing this one at the moment.
#1 solar unit with 20 tubes has 300
liters 
of
water and 20 wet thermosyphon tubes.
300 litres of water divided by 20 solar heating tubes gives a
ratio of 15 litres of water to one solar tube.
The unit works ok but 300 litres is a lot of water to be heated
by only 20 solar tubes.
Would be good in a much hotter country or a hot part of New
Zealand such as Nelson area with a lot of sunny days
The second unit #2 has 30 tubes. It
looks like this and uses dry HEAT PIPE tubes. 
There is no
attached hot water cylinder The usual recomendations for solar
hot water in New Zealand are one solar tube can heat 10 litres of
water so for this 30 tube solar I could use a 300 litre hot water
cylinder.
This solar unit is good for DIY people. There is no water in the
glass tubes. It has a stand for the ground or it is easy to make
your own stand. using treated wood or aluminium. Because there is
no water in the tubes they are not heavy
These units use the HEAT PIPE tubes which can withstand freezing weather down to minus 20 degrees centigrade according to the sales literature. Tubes are dry If a tubes is broken no hot water pours out and, in Summer you can easily remove some tubes to prevent overheating. This 30 tube unit complete with tubes cost me $1200 in 2010.
These types of pumped solar units are
used all over the world. You can buy the tubes in New Zealand,
build your own frame from aluminium or wood and make the manifold
unit or you can buy the whole thing 20 tubes for approximately $1500
plus you need all the accesories about $2000 all upplus a new hot
water cyclinder if your old one is not suitable plus installation
This unit needs a water pump. You could
use a solar panel to power a pump. Overseas a lot of people do
that. Another way is to mount the complete solar unit on the
ground and have an intermediate hot water cylinder higher and
heat the water by thermosyphon which does not require an electric
pump. That way you have a cylinder full of hot water similar to a
thermosyphon unit but without its serious problems and it is
possible to make the whole thing yourself.
These DRY heat pipe units are used all
over the world and have good advantages
There is no water in the tubes
The manufacturers say they will be ok down to minus 20 degrees
centigrade
They are light in weight and easily fit on your roof or on the
ground
You can have a lot of tubes for Winter and remove some in Summer
to control overheating. The solar unit will still work even if a
tube is broken.
Can be used with high or low pressure hot water systems
A major disadvantage is they do not heat very well in overcast
conditions. The reason is the thin copper tube in the center of
the glass collector contains a liquid which boils at 30 degrees
Centigrade, changes to a gas, rises up the tube and transfers
heat to the manifold but if there is low energy from the sun on
an overcast day in Winter the solar heating is almost nil and you
will need backup from electricity or gas or a wood burner.
-----------------------------------------
Here are some solar experiments I have tried looking for a cheap
solar hot water system.
..
Charging a car battery using a small solar panel works very well. Charging current is about 1/4 amp which is quite low. This low level charging keeps a charge on the battery in case you do not drive often. Some 12 volt battery solar panels plug into the cigarette lighter charging the battery without having to disconnect the wiring or remove the battery.
Using mirrors to concentrate sunlight
does work it is commonly used overseas to generate electricity
Here I am just trying to heat water in a pot with some small
mirrors The pot had to be moved to follow the sun. Some DIY
people overseas use mirrors to get heat with good success one
person on the internet melts lead with 54 x 6 inch square mirrors
in a very neat and compact frame. The mirrors only work with
direct sunlight not on a cloudy day and you have to move the
mirrors or the water. If you have an area with many sunny days
such as Blenheim or Nelson this type of electric power plant
would be good. The mirrors are cheap, the mirror positioning
equipment is not too expensive and the technology is being used
more and more overseas
The parabolic reflector worked very well giving over 100 degrees
centigrade in a few minutes at the focus which was a stainless
steel outside of a thermos flask. Had to move the reflector every
few minutes to follow the sun. It takes quite a lot of work to
get any sort of an accurate parabolic shape. This unit only works
in full sun not overcast sky.
Four heat pipe glass solar collector
tubes with homemade manifold. Gets very hot works with clear
skies but not as much with overcast skies. Heats water for a
paddling pool.
Insulated box with double plastic cover gets very hot in the sun
heats 100 metre x 12mm black food grade plastic pipe full of
water. Same as leaving your hose in the sun but a little better.
This unit gets hot in full sun but very little heat in overcast
weather. Plastic pipe softens when hot. Lots of people try this
on the internet because it is cheap. Major improvements are the
pipe should be continously rising. The pipe should be in firm
contact with a metal such as aluminium painted black to pick up
as much solar heat as possible. If you do that you get real heat
into the water but you need a pump moving the water otherwise it
can easily get too hot for the plastic pipe. People overseas try
this with plastic water pipe. some experimenters cover their
garage roof with pipe. I tried this but could not get solar hot
water in overcast conditions. In summer with full sun the hot
water was not bad.
All experiments were quite good in full
sun but not very good in overcast conditions except for
thermosyphon glass solar tubes which are surprisingly good at
heating water in overcast conditions. Thermosyphon wet 
tube which
contains 3 litre of water. These tubes heat well going up to 50
degrees centigrade in 4 - 5 hours on overcast Auckland days.
Clear skies give most of your solar energy, Overcast skies give
much less solar energy. You get a lot more solar energy in Summer
than in Winter.
In Winter the sun is at a much lower angle in the sky than in
Summer so hedges can block solar energy also trees and other
buildings and in Winter the sun rises later and sets earlier than
Summer so there is less solar radiation in Winter.
Cold water from the council water system is a lot colder in
Winter than in Summer. Good idea to insulate the hot and the cold
water pipes. In Winter the water starts off a lot colder and
there is far less solar enegy in Winter so it is hard to get hot
solar water in Winter.
Here I am trying out a small plastic
house for one tube to see if it gets hotter than the one right
next to it. After testing for a month there is a one or two
degrees advantage with the glasshouse heating the glass solar
collector inside but considering the time and effort and cost the
extra one degree is not worth it and the glasshouse uses up a lot
of space.
I will put a question mark against this one because I feel sure a
glasshouse would increase the solar energy to the tube not just a
small amount but a lot but I do not have a glasshouse at the
moment so I cannot do proper testing..
.
In New Zealand two types of solar
collectors work well. The older flat panel units work well in
direct sun and give some output with overcast skies but, because
they do not track the sun, they collect most energy when facing
the sun usually aranged to be noon. At other times the sun is at
an angle to the collector and , if it has a glass cover, some
sunlight is reflected away by the glass. They may work a lot
better with a plastic cover which is not as reflective as glass.
Glasshouse plastic cover works very well and keeps the weight
down and the cost down. A home made 2 metre high by one metre
wide flat plate collector using copper pipes and aluminium
collector plates and double plastic cover will give you some very
good hot water for less than $300 - $400 or you can buy flat
panel units on trade me from time to time. You have to take
precautions against frost or the pipes will burst. Some people
overseas use plastic (PEX) water pipe instead of copper nearly as
good, cheaper and not the same problems with frost breaking the
pipes. For a DIY person this is a good solution there are
hundreds of designs with construction information on the internet.
The main worry is freezing. Some ways to solve that are drain
back where all the water drains back into the hot water cylinder
and there is no water in the solar collector or the pipes so
nothing to freeze. Or use plastic or silicon pipe. Or use a
glycol anti-freeze mixture but then you need a copper coil heat
exchanger in your hot water cylinder. Solar flat panels have been
used for many years. There are hundreds of examples on the
internet
The other type of solar collector which works well in NZ is the
glass tube solar collector. These work very well in full sun and
the heat pipe tubes can withstand freezing conditions the sales
brochures say down to minus 20 degrees centigrade. These units
are made mostly in China and are more a factory product however
you can buy the glass tubes and make all the rest yourself
cheaper than buying one.
People have tried to get more heat from glass solar collectors various reflectors placed behind the solar collector tubes. Tubes with fixed or sun tracking reflectors give more solar energy with extra cost and usually reflectors need direct sunshine to reflect sunshine onto the tubes. As time goes by the reflectors get dirty and do not work very well
Simple arc fixed reflector. Wider than
a glass tube reflects some sunlight onto the back of the glass
tube
CPC fixed reflector. Shaped to reflect angled radiation, direct
radiation, and diffuse radiation
Sun tracking reflector. Parabolic sun tracking reflector
Angled fixed reflector. Designed to put more radiation on the
solar collector tubes.
The trouble with reflectors is they get dirty and do not reflect
heat as intended.
SOLAR HOT WATER IN WINTER
In most parts of New Zealand you can
get plenty of Summer solar hot water but you cannot rely on solar
hot water in Winter. All over the world in countries with
definite Winters this is the same problem. How to get usefull
solar hot water in Winter. Seasonal store methods are being
investigated but nothing on the market for an individual house
yet
In Winter you can go for days with very little sunshine just grey
overcast rainy days. You have to switch on the electric water
heater to get hot water. Often after a hard frost there are clear
skies with good solar hot water temperature close to 60-70
degrees centigrade. On a heavy overcast day or a rainy day the
solar water temperature in Winter generally does not get above
lukewarm about 30-40 degrees centigrade. See if you can get a
meterological report for your area for on average how many Winter
days are raining, snow, overcast or sunny. Then you can start
thinking is it worthwhile to get solar energy.
There are 3 ways being investigated overseas to store Summer solar hot water for use in Winter. Try Google seasonal storage.
(1) Store ordinary hot water in a very well insulated hot water cyclinder or box made to store hot water. It is possible to use very good manufacturing methods and good insulation to store hot water with a very small daily loss of heat. Needs a lot of space to store a lot of hot water but you do not need to store an entire Winter of hot water. Maybe a few days or a week would be ok to cover the times when it is raining or not able to collect your own solar hot water. Something like a metre cube which is about 1000 liters of water. You have to have a larger solar collector because you are collecting your normal amount of solar hot water plus collecting extra for rainy days.
(2) Latent heat. Latent heat is where a material such as parafin wax or gluaber's salts is solid at one temperature but changes to a liquid at a different temperature. A lot of heat is stored in latent heat. By using latent heat you can maybe reduce your heat storage volume by half. But you will have to buy the latent heat chemicals.
(3) Chemical heat. Using chemicals such as magnesium sulphate (epsom salts) and others, it is possible to store heat for a very long time weeks, months or years with no loss of heat with a volume reduction compared to hot water storage of 1/9 th and no deterioration of the chemicals. The process is not as simple as storing hot water in a hot water container but a great deal of research is being done to find a simple and effective storage system suitable for home use. Google seasonal storage magnesium sulphate.
===================
Everyone knows in New Zealand solar hot
water units are good in the summer but not so good in Winter.
This is a serious limitation with solar units because after you
have bought a solar unit and paid for it to be installed you
still need to use expensive electricity to heat the water in the
Winter on days when there is not enough sunshine. In fact it is
not such a serious problem because you can get solar hot water
Spring Summer and Autumn nearly every day but in Winter you have
to use some electricity to heat the water on perhaps half the
days in Winter. Might not be very much to worry about.
The wet thermosyphon tubes have fairly serious problems when
fitted in a factory built solar unit as mentioned previously,
They are perfect for a stand alone solar heater but have some
drawbacks for connecting to a home hot water system. but if you
just use a wet thermosyphon tube on its own they are good at
heating water even on an overcast day.
When used on their own without being attached to a usual solar
hot water cylinder they heat water very well. Each tube by itself
holds about 3 litre of water. I have found over 12 months testing
the heated water temperature for 9 days out of 10 is at least 50
degrees centigrade. All through Winter even in heavy rain the
tubes get up to 50 degrees centigrade That is still usable hot
water. The slightest bit of sunshine makes the temperature shoot
up to 60 degrees or 70 degrees centigrade. Auckland area
The results of 12 months testing nearly every day using two
commercial solar hot water units #1 and #2 in my opinion they
both heat water but each unit has problems
#1 solar unit using thermosyphon tubes has several serious
problems regarding freezing in Winter and loss of all water if
one tube is broken. It is sluggish to heat,, the hot water
cylinder does not hold the heat very well overnight in Winter.
#2 solar unit heat pipe phase change tubes has very good points, is used all over the world and works very well on a sunny day but not very good on an overcast day.
A third type using U tubes was not tested
A forth type just one thermosyphon tube holding 3 liters of water tested most days for 12 months
The forth type was the best by a long
way. The reason is instead of one tube being used to heat 10 or
15 litres of water as in a commercial solar unit, I am using one
tube to heat 3 litres of water.
Because of the considerably smaller amount of water the tube
heats water much faster than a comercial solar heater and also
heats water even on overcast days and on an overcast day if the
sun comes out it heats water quickly. Every day I can count on 3
litres of hot water per tube. The temperature varies between
minimum 40 degrees centigrade if it is raining, -- up to 100
degrees centigrade in full sun. Thats in the middle of Winter. On
a clear full sun day in summer I can get three heatings per day
to 100 degrees centigrade. Another outstanding feature is; say on
an overcast day in the middle of Winter, the solar hot water
temperature is around the 40 degrees centigrade but if there is a
short period of sunshine, the solar water temperature starts to
rise rapidly.
At the moment I fill each tube using a metal watering can and,
when the water is hot I tip the water into the metal watering can
and take it inside. It is not nearly as convenient as turning on
the hot water tap
If you do not need to use the hot water leave it in the tubes. If
the tubes get too hot and boil cover with shade cloth the type
used in gardens to reduce sunlight on plants.
These glass tubes can break if you are not careful. Be especially
careful so no one can fall onto the glass tubes a fence or
netting keeps children away from the tubes, mine are locked away.
You can empty one or all tubes if you need the hot water and
refill with cold water straight away without breaking a tube. If
you leave an empty glass thermosyphon tube out in the hot sun
they get very very hot and you risk breaking the tube filling it
with cold water because of thermal shock to the glass. If I am
not refilling straight away I put the tubes in the shade to cool
down
At the moment I fill the solar glass tubes with a metal watering
can and empty the hot water into the watering can by hand. I can
improve this arrangement to give easier filling with cold water
and removal of hot water so I don't have to pick the tubes up and
tip them into the watering can. This will be a very simple and
useful and inexpensive type of solar hot water unit that heats
water faster and to a higher temperature because each tube is
only heating 3 litres of water instead of the usual 10 or 20
litres it tries to heat in a factory made solar unit.
Using the thermosyphon tubes in this way gives much better and
much cheaper Winter solar heating and also removes several
serious problems with these thermosyphon tubes. For example you
can easily make a solar water heater using as many tubes as you
like at low cost and if a tube is broken it only empties the
water in the tube not the whole lot. Tubes are easily replaced.
If freezing is likely which is a very serious problem for glass
tubes filled with water, you can check with a temperature sensor
and tubes can be pumped dry.
As a DO IT YOURSELF project I am very happy with these thermosyphon tubes for our own solar hot water. When we want the hot water it is a simple matter to tip a tube into a metal container take it inside and use the water. This is like going back to the old days and abandoning modern hot water plumbing but I have a lot of fun and when your electricity bill becomes high enough you will look at every alternative. I have found after 12 months testing these simple tubes heat water very well. They are clean and simple and hygenique and only require some labour to move the hot water indoors where it is needed and of course people can easily solve those problems with no difficulty.
IS THIS SUITABLE / ECONOMICAL FOR HOME SOLAR HOT WATER
Our hot water cylinder is 140 liters. for full solar hot water almost 100 percent reliable in all weather I would need 140 divided by 3 = at least 46 tubes at $30 per tube = $1380 plus pump, sensors, control unit, stand, copper pipe, insulation, safety valve, mixing valve, plumber for a total about $2500. Our power account is about $150 per month roughly $70 for hot water= $840 per year so the payback period would be about 3 years. I am almost happy with this. The cost is ok. The payback time is ok. 46 tubes is a lot of tubes and that is for a 140 litre hot water cylinder which is not a very large cylinder. It would be better if I can get the tubes down to perhaps 20. The main factor is how much hot water we use for the shower.
I unscrewed the drain fitting on the shower and ran the water into a large plastic water tank and after showers I measured the water used = 60 litres for two people. I estimate 2/3 hot water and 1/3 cold water and our showers were 10 minutes each so 40 litres hot water seems reasonable. That means at a pinch we could get by with 12-15 tubes for the shower plus a further 5 tubes for everything else total 20 tubes. estimating cost of tubes at $30 each = $600 which is a lot better than 46 tubes.
The main cost is getting the solar hot water into the hot water cylinder for use in the shower. If we could find another way to have a shower witout altering the existing hot water system that would be good. Something like a second hot water cylinder outside and a pump to give a pressure shower for 10-15 minutes We would not need to touch the household hot water system.
So how to have a good shower with unpresurised solar hot water costing very little is an interesting excercise.
As well as solar hot water for washing the dishes and showers it should be possible to cook in Summer with very hot solar water.
In all my tests i have been using some black glass solar tubes and some slightly blue glass solar tubes with red interiors. The blue tubes are miles better. Side by side they heat faster and get to a higher temperature than the black tubes.
There are several things to consider about a DIY solar unit. Even if the unit preforms very well it must be safe and not endanger yourself and other people and must meet New Zealand standards for solar glass tubes. Regulations say the hot water cylinder must reach 60 degrees centigrade at least once a day because of possible legionaires disease.
So what I am thinking is fill 20
thermosyphon tubes with water in the morning.. Let it heat all
day pull the shade cloth over if heats more than 90 degrees
centigrade. In the evening tip the tubes into a hot water
cylinder fitted with a 50 degree tempering valve..
Next day pump that 50 degree hot water to the bathroom and have a
shower. Fill the tubes and start again. At night the tubes are
empty so cannot freeze
20 Solar tubes $30 each
80 litre hot water cylinder --- had one
12 volt Marine water pump $150
12 volt battery -- Had one
12 volt Solar panel to charge battery --- Had one
20 mtres plastic hot water pipe -- $30
Tempering valve --- $70
Shower head --- Had one
Shower mixer --- Had one
For about $1000 I have hot water every day 365 days of the year.
--------------------------------------
I have been thinking about another idea which is to buy a 400-500
watt solar panel and connect the solar panel to an inverter which
powers the heating element in my hot water cylinder.
The reason I am thinking about this is to lower my overall water
heating costs. By using solar heated hot water and connecting it
to my house hot water cylinder I have quite a number of
moderately expensive plumbing stuff to buy. A fairly long run of
copper piping, thermal insulation for the copper pipe, copper
fittings, temperature mixer. Pressure and temperature safety
valves, a registered plumber to finish the connections and sign
it off as legal and other costs so getting hot water into the
household hot water cylinder is not going to be cheap and in the
cold Winter I am probably still needing to use some electricity
to heat the water for a shower. Its important to get solar hot
water or however you heat your hot water at a price that is
called a payback time that is if you pay an excessive price your
payback time is too long maybe 10-15 years you might as well use
the money for something else and pay the electricity costs for
the hot water. On the other hand if you can install solar hot
water at a price that gives you a payback time of 3-5 years you
might be happy with that.
In New Zealand hot water electrical circuits are on a meter to
measure how much electricity is used and the property owner/tennant
as well as paying for electricity used also has to pay a monthly
rental for the meter. This means if your solar hot water does a
very good job of heating the water in the Summer but not in the
Winter, you have to keep the electricity supply which also
includes the meter and the meter rental and you have to pay that
rental even when in Summer you never use any electricity to heat
water at all you still have to pay for rental of the meter.
But if i do not use any solar hot water equipment and instead buy
a 400 watt solar panel and run it throught an inverter ( even a
cheap square wave inverter would do ) then when the sun is out
the panel heats the water. My hot water cylinder has a 1000 watt
element and runs for about 2 hours from 11pm at night. (night
rate) to heat 136 liters of water. We do not use that much hot
water so it is only bringing the water we have used during the
day back up to 60 degrees thats where the thermostat is set to.
This would be a very simple possibility not requiring any
batteries or much control equipment and all I need is to wire the
solar panel to the inverter and it will heat the hot water
cylinder all day until the thermostat turns it off when the water
is hot enough.
This is a good idea apart from one small problem. solar panels in
NZ are expensive. Much more expensive than overseas. Suppliers
charge whatever they think customers will pay. They have been
priced about $10 per watt. A 400 watt solar panel would cost me
about $4000. I can get a solar panel much cheaper on Ebay USA
even including postage although GST may have to be paid. I am
looking for a 400-500 watt solar panel for a lot less than $4000
more like $1000-$1500. I can get some good German panels in
Australia for that price plus postage to NZ.
The reason I think this is a good idea is because of the
outstanding simplicity of installation compared to a full solar
hot water installation . If there is enough light for you to see
across your section there is enough light for your solar panel to
produce some electricity and the more light the more electricity
and all you need is some wire, an inverter and your solar panel.
You don't even need an inverter because several companies sell
heating elements for hot water cylinders at 24 volts 48 volts and
almost any other voltage you require So it is an easy matter to
unscrew your existing 230 volt heating element and scew in a
heating element suitable for the solar panels you buy. These
lower voltage heating elements cost between $100 and $300 depends
on the wattage.
To take into account low light conditions I should be looking at least 1000 watts of solar panels probably 1500 watts but thats a lot of cost and would need quite a bit of area. The main problem is the very high cost of buying a solar panel in New Zealand which are about $10 per watt. Just recently I have seen extra good solar panel prices at DISCOUNTSOLAR $3-$6 NZ per watt including GST in the range 10 watts to 90 watts and the 90 watt units are certified to IEC61730. Some Australia solar panel supliers has German solar panels at prices close to $2-$3 AU per watt. I probably would not buy one 400 watt solar panel. More like 4 x 100 watt panels in case one failed.
Unfortunately the efficiency of solar panels is very much lower than the efficiency of solar hot water tubes so I would need not only quite a lot of money to buy enough solar panels but also use a lot more area for solar panels than for solar hot water tubes. The greater area is not a problem I could put these panels on my garage roof but the high cost of the panels (2011) is the main drawback but I will certainly keep an eye on solar panel prices they are saying 2012 solar prices will drop to $1 per watt in USA .
......
Richard Williams
remp@ps.gen.nz
See my other articles at
http://cobournereserve.atnz.net