Low Emission Water Heating Technologies

8/8/2019 Low Emission Water Heating Technologies

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Low Emission WaterHeating Technologies

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Factsheet

a joint initiative of the Australian,

State and Territory Governments

Low Emission Water Heating Technologies 1

Solar water heating

What is a solar hot water system?

A solar hot water system uses energy from the sun to heat water for your home. Solar collectors

on your roof absorb the suns rays and heat the water, which then flows to a storage tank, ready

for use.

A solar hot water system is an effective way to make use of Australias abundant sunshine.

Depending on your location, the direction your solar collectors face and the amount of water you

use, a solar hot water system can provide between 50 per cent and 90 per cent of your hot water

needs. Solar hot water systems come with electric or gas boosters to provide the rest of your hot

water needs.

There are many different types of solar hot water systems available. It is important to select

the type most suitable to your family size, climate, house type, roof characteristics (especially

orientation), water quality, available space and visual style.

How does it work? A solar hot water system uses a tank to store water that has been heated by the sun.

Cold water flows from the tank to a solar collector, usually positioned on your roof. In a

split system like the one shown in Diagram 4, cold water is pumped up to the collector.

In a thermosiphon system (diagrams 2 and 3) with the tank above the collectors on the

roof, cold water flows naturally into the collector because it is heavier than hot water.

The solar collector is made of materials that absorb the suns heat very efficiently.

The cold water travels through the collector and is heated before flowing into the tank.

Hot water rises to the top of the tank and colder water is taken from the bottom and

returned to the solar collector. When you turn on your hot water tap, water is taken

from the top of the tank where it is hottest.

Most solar hot water systems have a boosting system to make sure that water is heated

to a temperature that inhibits the growth of the Legionella bacteria and guarantees hot

water supply when there is insufficient solar radiation. The booster may be set to turn

on automatically or can be set up with a manual switch so the householder can control

the boosting of the system, depending on their needs.


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2 Low Emission Water Heating Technologies

Where should collectors be positioned?

The ideal location for placement of solar hot water system collectors is one that directly faces

the sun at all times. A north-facing roof is ideal for Australian locations. The best tilt angle of the

collectors for all-round performance will depend on the latitude of the location where the system

is being installed (see Diagram 1).

Diagram 1. Most efficient latitude angles for panel placement in Australian capital cities

Latitude angle

Equator - ie. north in

the southern hemisphere

Solar Collector

Location

Darwin

Brisbane

Sydney

Melbourne

Adelaide

Hobart

Perth

Canberra

Latitude Angle

12.5

27.5

34

38

35

43

32

35.5

For solar hot water system collectors, the Australian Standard (AS 3500.4) states that the

orientation of the collectors can vary from:

between 50east and 70 west (Victoria)

between northeast and northwest (other states).


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Solar hot water systemswhat are your options?

Thermosiphon systems

Thermosiphon systems use a basic scientific principle to keep water circulating through a hot

water system, the thermosiphon effect (see Diagram 2). Water moves according to temperature

differences within the hot water system: cold water flows downward in the collector and as itheats up it becomes less dense and rises up through the collector. If the storage tank is installed

directly above the collector, the heated water will flow into the tank ready to be used.

Integrated or close-coupled thermosiphon systems (roof-mounted)

Collectors and tanks are mounted close together on the roof, with the tank mounted above

the collector to take advantage of the thermosiphon effect.

Advantages Disadvantages

Very low level of heat loss

Suits homes with no ceiling space

Provide good, steady water flow

Operate even when mains power is down

Only one installation site

Difficult to install due to the weight

of the storage tank

Roofs may not be strong enough to support

the whole system and may require additional

bracing or an engineers report

Some property owners dont like the look

of a fully roof-mounted system

Diagram 2. Workings of an integrated or close-coupled thermosiphon system (roof-mounted)

Heated water

Cold water inlet Heater water to

internal fxtures

Roo-mounted

solar collectors

Roo-mounted

hot water

storage tank


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Split or remote thermosiphon systems

Collectors are mounted on the roof, with the tank installed inside the roof cavity due to space

restrictions, weight concerns, or aesthetic reasons. The tank is still above the collector vertically

to take advantage of the thermosiphon effect.


Very low level of heat loss

Tank is neatly hidden from view inside

the home

Suits home with very little roof space

Provides good, steady water flow

Operates even when mains power is down

Installation is more complex working both

inside and outside the home and inside

the roof cavity

Leaks from the storage tank may find their

way inside so a spill tray and drainage line

is needed to remove the water.

The roof cavity may not be strong enough to

support the whole system and may require

additional bracing or an engineers report

Diagram 3. Workings of a split or remote thermosiphon system


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Non-thermosiphon systems

Non-thermosiphon systems position the tank and collectors separately. A pump is used to

circulate the water through the system.

Split systems with forced circulation

The collectors and tank are mounted separately, usually with the tank on the ground.

An electric powered pump is used to circulate the water through the system.

The most common collector used is the flat panel collector.


Suits homes with very little roof space

Tank is easy to maintain when it is

at ground level

Tank may be placed in the same location

as the one it is replacing

Allows collectors to be retrofitted to an

existing water tank

Tank can be hidden from view

System cannot operate effectively unless

mains power is available to power the pump

Installation can be more complex when

there are two installation sites (the roof

and the ground)

Some heat loss will occur due to tankand panel separation

Diagram 4. Workings of a split system with forced circulation

Roo-mounted

solar collectors

Hot water storage tank

Heated water to

internal fxtures

Cold water inlet

Pump

Power

outlet


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Split system with evacuated tube collectors

The collectors and tank are mounted separately, usually with the tank on the ground.

An electric powered pump is used to circulate the water through the system.

The collectors used are evacuated tube collectors.


Evacuated tube collectors heat water using a

combination of glass tubes, a vacuum and a

heat transfer fluid. The vacuum means there

is less heat lost to the outside this technology

is more efficient in cooler temperatures

Curved shape of the tubes means they can

collect the suns heat for a greater proportion

of the day than flat plate collectors

Suits homes with very little roof space

Tank is easy to maintain when it is at

ground level

Allows collectors to be retrofitted to

an existing water tank

Tank can be hidden from view

System cannot operate effectively

unless mains power is available to

power the pump

Installation can be more complex

when there are two installation sites

(the roof and the ground)

Some heat loss will occur due to

tank and panel separation

Diagram 5. Workings of a split system with evacuated tube collectors

Evacuated Tube

Collector

Gas Booster

Hot Water to House

Gas line

Cold water inlet

Pump

control

Power

outlet


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Boosters

A solar hot water system can meet most of your households hot water needs. On days where

hot water usage is higher than usual, or on cloudy or rainy days, water stored in your tank may

need an additional boost to maintain your preferred water temperature.

The booster heating unit will only activate when the water temperature is below the thermostat

setting and will automatically turn off when the temperature reaches the thermostat setting.

For further control, a manual on/off booster switch can be installed in a convenient location,

such as the kitchen or laundry. Boosters use an alternative source of energy, such as electricity

or gas, to heat the water.

Consider the option of a manual on/off booster switch so you can control the level of boosting to

your system. This gives the option of switching off the booster in summer or when you are away.

Timers can also be used to ensure the booster is used as little as possible to maximise the

solar contribution.

There are a number of ways to boost a solar or heat pump hot water system. The two most

common are electric boosting and gas boosting.

Electric Boosting

One or two electrical elements are immersed in the storage tank and controlled by a thermostat.

When the temperature drops to a set point, the elements are activated.


Electric boosting is simple to install and uses

much less power than a conventional electricstorage hot water system

Costs can vary depending on whether the

electric boosting is supplied at the retailersday rate or off-peak rate. If it is the day rate,

then the cost for boosting can be expensive;

if off-peak rate, the system may not be able

to boost as required

Gas Boosting

Type 1: A gas burner is installed with the storage tank and controlled by a thermostat.

When the water temperature drops, the burner uses gas to heat the water.

Type 2: A gas burner is installed in-line in the hot water pipe between the storage tank and

the supply point. This heats the water instantaneously depending on hot water needed.

Both can use either natural gas (mains supplied) or LPG.


Gas supply can be provided either from

reticulated supply or from bottled LPG gas

Gas has lower greenhouse gas emissions

than mains electricity

Gas has better performance efficiency thanelectric water heating

Reticulated gas supply is not available

in all locations

Bottled gas may be expensive, especially

where there is high consumption (eg.

cold climates or high hot water use) Gas-boosted systems are more expensive

to buy than electric-boosted systems


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Frost protection

If you live in an area that experiences temperatures below four degrees Celsius (4C) you will

need to consider frost protection for your solar hot water system. As the temperature of water

approaches freezing, it begins to expand. This expansion is significant below 4C. Expansion of

water and ice inside the solar collector can damage the solar collectors.

Ask your installer about frost protection for your new water heater.

There are several types of frost protection available for solar hot water systems.

Frost protection valves

Also known as frost dump valves, frost protection valves use a heat sensitive metal element

that releases a valve when the temperature falls below 4C. This relieves the pressure caused

by water expansion and encourages water circulation, which warms up the collector.


Prevents water from freezing in collectortubing and rupturing it

There is some water wastage and decreasein efficiency caused by dumping of water

from the collectors

Frost protection valve can fail

Pump circulation protection

As in a split system, an electric pump continually circulates water through the tank and collector.

This means that warm water from the tank reaches the collector and prevents freezing.


Protects the collector tubing from exposure

to water freezing and rupture

Uses power for the pump to work

Pump can fail

Antifreeze

Glycol based anti-freeze (as used in car radiators) is a common solution in heavy frost or snow

prone areas. The antifreeze is kept separate from the potable water supply through the use of

a heat exchange system.


Provides excellent frost protection More expensive than solar hot water systemswithout glycol frost protection

Slightly less efficient than systems that

directly heat the water

System Draining

Once the ambient temperature drops below a prescribed level, a valve opens and the collector

is drained of water.


The valves can be operated manually

or automatically

Heated water in the storage tank is retained

Valves can fail if not serviced correctly


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Heat pump water heating

What is an air-sourced heat pump?An air-sourced heat pump absorbs heat from air and transfers it to heat water (see Diagram 6).

It runs on electricity but is roughly three times more efficient than a conventional electricwater heater. When used in the right environment they save energy, save money and reduce

greenhouse gas emissions.

How does it work?

A heat pump works on the same principle as a refrigerator, but instead of pumping heat out of

the fridge to keep it cool, they pump heat into the water. Electricity is used to pump a refrigerant

through the system. The refrigerant transfers the heat absorbed through the air to the water in

the tank (see Diagram 6).

The secret to making a heat pump work is the use of a refrigerant that evaporates at low

temperatures.

There are several steps in the process:

1. Liquid refrigerant passes through an evaporator where it picks up heat from the air

and becomes a gas.

2. The gas refrigerant is compressed in an electric compressor. Compressing the gas

causes its temperature to increase so that it becomes hotter than the water in the tank.

3. The hot gas flows into a condenser, where it passes its heat to the water and turns

back into a liquid.

4. The liquid refrigerant then flows into an expansion valve where its pressure is reduced,

allowing it to cool and enter the evaporator to repeat the cycle.

As long as the outside temperature is higher than the cold refrigerant the heat pump will absorb

heat and move it to the water. As the outside temperature decreases, this becomes more difficult,

which is why heat pumps dont work as well in places where temperatures are low.

Fresh air needs to flow across the evaporator to allow heat to be absorbed continuously.

A fan is used to assist air flow and remove the cooled air. A ventilated space is necessary

for the evaporator to extract heat effectively.

A heat pump uses electricity to drive the compressor and the fan instead of using electricity

to heat the water directly. The heat pump is able to transfer a much greater amount of heat

energy from the surrounding air to the water, which makes it highly efficient. The amount

of heat that is able to be transferred from the air to water depends on the ambienttemperature.


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Heat Pumps

Heat pumps consist of an evaporator, condenser, compressor and storage tank. They use the same

process as a refrigerator or air conditioner, but in reverse: to harness heat instead of remove it.


Require much less electricity than

conventional electric water heaters

Ground-mounted so no need for roof

space or roof strengthening

Can operate in most climates but are most

efficient in hot and humid conditions

Can be installed where shading

prevents solar hot water systems

from working effectively

For heat pumps to work, there needs to

be a difference in temperature between

the water in the system and the outside

temperature. In cold climates, without much

of a temperature difference, heat pumps

are much less effective

Can be noisy (similar to an air conditioner)

Are more efficient on day-rate tariffs

(which are more expensive than off-peak

tariffs), especially in colder climates

Diagram 6. Workings of a heat pump

Expelled cold air

Compressor

Condensing

Unit

Air intake vent

Cooled

Rerigerant(liquid)

return to

evaporator

Air heated

rerigerant(vapour) to

compressor

Hot Rerigerant

(vapour) fow to tank

Fan

Evaporator

Warm air in


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Diagram 7. Heat pump storage tank

Heat Pump

Storage Tank

Hot water Outlet

Tempered Water

to Appliances

Cold Water Inlet

Electronic Controller

TV

Heat pumps generally do not operate efficiently in areas that experience long periods of cold

weather. However, some heat pump models have a defrosting function to allow the system to

continue operating at cold temperatures. Although the heat pump might still function, it could

cost a lot to run and wont produce much hot water. In addition, heat pumps may need an

electric boosting element inside the tank to ensure a reliable supply of hot water during

periods of very cold weather. Check with your supplier whether these are features of the

heat pump you are considering.

Gas water heating

What is a gas hot water system?Gas hot water systems burn either natural gas, delivered via a piped gas (reticulated) network,

or liquefied petroleum gas (LPG), either bottled or, less commonly, reticulated, to heat water.

There are two main types of gas water heaters: storage and instantaneous (continuous flow).


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Storage hot water systems

Storage hot water systems heat the water stored in a tank. The systems are all very similar in the

way that they work. The main variations are in the tank material, the burner and flue technology,

and whether they are designed for indoor or outdoor installation.

The most common tank material is enamelled steel, but stainless steel is also used. Stainless steel

is more expensive, but the tank will usually last longer.

Some units use a special flue system to recirculate the hot flue gases around the outside of the

tank to increase heat transfer into the water and increase their efficiency.

Units installed indoors need a flue to exhaust the combustion gases outdoors.

Diagram 8. Gas storage system

Hot water out

Cold water in

Temperature

controls

Gas in

Thermostat

ignites gas burner

Flue


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Instantaneous (continuous flow) hot water systems

Instantaneous gas hot water systems, also known as continuous flow gas hot water systems,

dont have a storage tank and heat the water only when it is required.

There are two different types of instantaneous hot water systems. One type is very simple and

has a fixed burner flame so the water temperature will vary with the rate of water flow from the

tap. It usually has a pilot flame or battery ignition and does not need an electricity supply.

The other type has electronic controls that vary the flame size to deliver water at a constant

preset temperature and fans that improve the combustion efficiency. It can be fitted with

controls to allow you to set the water temperature you want. This means you can preset the

water temperature so you dont need to mix the hot water with cold water to ensure a safe

bathing temperature.

Electronic units can be preset to deliver hot water at no more than 50C, as required by

plumbing regulations for bathrooms.

Very efficient condensing instantaneous units are also available. They use the heat from the

exhaust gas to pre-heat the cold water before it enters the heat exchanger, thereby reducingenergy use. Although rated at 6 stars, currently the maximum rating, they use less energy than

non-condensing units. Look for the energy consumption figure (megajoules per year) on the

label as well as the star rating to compare products.

Diagram 9. Instantaneous gas hot water unit

Secondary Exchanger

(preheats incoming

cold water)

Water fow ignites

gas burner

Cold Water inGas in

Hot water out

Primary heat

exchanger

Combustion Gas

at 200C

Exhaust rom fue

aprox. 60C


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How do storage hot water systems work?

Storage hot water systems use a gas burner located underneath the water storage tank to heat

the water. They usually have a continuously burning pilot flame that ignites the main burner

when it is needed. Heat is then transferred from the gas burner through the bottom of the

cylinder. Some heat also flows from the hot flue pipe into the water. When hot water is drawn

off at the top of the tank, cold water enters the tank at the bottom. Most units have anadjustable thermostat to set the water temperature. When the temperature at the bottom

of the tank drops below the level set on the thermostat the pilot flame ignites the main

burner so the water is kept constantly hot. The energy from the pilot flame is not wasted

as it helps to heat the water in the tank.

Storage hot water systems should be set to at least 60C to kill any pathogens and avoid

Legionella, so you might like to consider a temperature control valve to limit the bathroom

water temperature. The hot water from the water heater needs to be mixed with cold water

to ensure a safe bathing temperature (50C to avoid scalding).

Storage hot water systems come in a range of tank sizes from about 90 litres to 300 litres.

Storage hot water systems lose heat through the walls of the tank (called standing heat loss),so they need to burn gas regularly to keep the water at the desired temperature. These losses

can be a significant proportion of your hot water energy use and your gas bill. All tanks have

some insulation to reduce standing heat losses, but a well-insulated tank will have lower

storage losses, so look for one with a high energy star rating.

All gas hot water systems except gas-boosted solar systems have an energy star label to tell

you how efficient they are. This is similar to the energy label on electrical appliances. The more

stars the better. Once you have decided on the type and size of water heater you need, use the

label to select the most efficient model by choosing one with the highest number of stars and

the lowest energy consumption. More stars mean lower running costs for you and less

greenhouse gas emissions.

Storage hot water systems have an energy efficiency rating between two and five stars.

In late 2010, state and territory governments will introduce a minimum performance standard

for gas water heaters that will require them to be at least four stars.


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How do instantaneous hot water systems work?

In instantaneous hot water systems, the water is heated by a gas burner as it flows through a

coiled pipe called a heat exchanger. The gas burner starts when a hot water tap is turned on.

It only heats the amount of water that is required instead of continuously heating a full tank.

As they dont store any hot water, they do not have heat losses from a tank and so can have

energy and cost savings.

To allow enough time for heating, the water is slowed down as it flows through the heat

exchanger. This means that instantaneous units supply hot water at a lower pressure than storage

systems. They can typically deliver hot water at flow rates per minute of between 10 litres and

30 litres, depending on the model. Some instantaneous systems have a pilot flame to light the

main burner. Others have electronic ignition that uses a spark to light the gas, the same as those

used on gas stoves. Instantaneous units with a pilot light are less energy efficient than those with

electronic ignition, because, unlike in a storage system, the pilot light energy is wasted. Look for

models with electronic ignition.

Instantaneous units have an energy efficiency rating in the range of four to six stars. The energy

label assumes that you use 200 litres of hot water per day, which is about what a typical four

or five person household would use. If you use that much hot water, then a five-star storage

system and a five-star instantaneous system will use about the same amount of energy each year.

However, if you use less hot water, an instantaneous system might use less energy as it has no

storage losses.

Gas hot water systems

High efficiency gas hot water systems can be installed regardless of the solar availability

and ambient temperature. They may be the best option when a property is not suitable for

either solar or heat pump hot water systems.


Are less greenhouse-intensive than

conventional electric hot water systems

Instantaneous hot water systems can supply

unlimited hot water

Generally have lower upfront costs than

solar or heat pump hot water systems

Require access to piped or bottled gas.

This is not always available, or can be

expensive in the case of bottled gas

Instantaneous systems can result in

increased water use due to water having

to pass through the water heater to

ignite the burner

Unlimited hot water may result in

increased energy costs if greater amounts

of hot water are used than previously


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Conditionsor

Technology

Closeco

upled

thermosiphon

Remote

thermosiphon

Flatpanel

splitsystem/

forced

circulation

Evacuated

tube

Heatpump

NaturalGas&

LPGSystems

Hotand/or

humidclimate

Notrecom

mended

Collectors

arenot

designed

forhigh

temperatures

Notrecommended

Collectorsarenot

designedforhigh

temperatures

Yes

Yes

Yes

Yes

Temperateclimate

Yes

Yes

Yes

Yes

Yes

Yes

Coldclimate(e.g.

susceptibletofrost)

Yesifglycolor

highefficiency

collectors

used

Yesifglycolor

highefficiency

collectorsused

Yesifglycolor

highefficiency

collectorsused

Yesprovide

optimal

performan

cein

coldclimates

Considersystem

specifically

designedfor

colderclimates

Yes

Non-idealsolar

conditions(e.g.

shading,bad

orientation,not

enoughroofspace)

No

No

No

No

Yesdepending

onclimateand

situationof

waterheater

Yes

Issueswithaesthetics

orweightonroof

Yes*

Maybe

No

No

No

No

Abletoretrofitto

existingroof

No

No

Yes

Yes

Notapplicable

NotApplicable

Licensing

Plumbing/

electrical

Plumbing/

electrical

Plumbing/

electrical

Plumbing/

electrical

Plumbing/

electrical/

ARCTIC

(Refrigerant)

Plumbing/

electrical/

gasfitting/

lpgasfitting

*Thereareissueswiththeweightandtheaestheticsofthistechnologytypeforon-roofin

stallations.

Theremaybeaestheticsissuesandweightiss

uesintheceiling.

Therearenoweightissues,butpossibleaesth

eticissues.

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