Beechworth Urban Landcare and Sustainability

Choosing a solar hot water unit can be a confusing experience for the average consumer. There are many types available and many traps for the unwary. This fact sheet hopes aims to provide some basic information to assist.

Solar hot water uses the energy of the sun to heat domestic hot water. They can provide up to 85% of domestic hot water energy over the whole year. All systems require an auxiliary energy source during winter, this typically being an electric immersion heater in this region (where gas is available this is a better option). Solar hot water is probably the most cost effective way of reducing energy consumption in the home.

There are two main types of solar hot water system – compact and pumped systems. There are also two main types of absorbers – flat panel and vacuum tube. The flat panel absorber can run directly with water or, as is more common these days, with a mixture of antifreeze and corrosion inhibitor which heats the water via a heat exchange system. This avoids problems with frosts damaging the system and extends the life of the unit considerably.

Compact Systems (Passive Systems) These are also known as thermosiphon systens and consist of a tank for the heated water, a solar collector and the connecting pipes all pre-mounted in a frame. The tank needs to be positioned higher than the collector panel as the hot water will rise to the tank without assistance. This requires no pumps.

This is the most efficient system as it requires the least amount of electricity either for pumping or for auxiliary heating. As the weight of the entire unit including the 300litres of water may require reinforcement of the roof, this needs to be considered, especially if retrofitting. It is probably the easiest system to retrofit if this is not an issue.

Pumped Systems (Active Systems) Active systems require pumping as the tank and auxiliary heating system is remote (an lower than) the collector system. This pumping requires energy and can reduce the potential energy savings by 10-20%. If you include a solar powered pump this eliminates this but increases the cost considerably.

Types of Thermal Collector

Flat Plate Collector A flat plate collector consists of a thin absorber sheet (of thermally stable polymers, aluminium, steel or copper, to which a black or selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with a glass or polycarbonate cover. Fluid is circulated, using either mains or solar electricity, through the tubing to remove the heat from the absorber and to transport it to an insulated water tank, sometimes directly or otherwise to a heat exchanger or to some other device for using the heated fluid.

Evacuated Tube Collector Evacuated tube collectors are made of a series of modular tubes, mounted in parallel, whose number can be added to or reduced as hot water delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flatplate collector). The tubes are covered with a special light-modulating coating. In an evacuated tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it. The absorber can either consist of copper (glass-metal) or specially-coated glass tubing (glass-glass). The glass-metal evacuated tubes are typically sealed at the manifold end, and the absorber is actually sealed in the vacuum, thus the fact that the absorber and heat pipe are dissimilar metals creates no corrosion problems. The better quality systems use foam insulation in the manifold. low iron glass is used in the higher quality evacuated tubes manufacture.

The quality of the tubes can vary considerably. Cheaper boro-silicate glass tubes can shatter during extreme heat and can also have long term problems with corrosion.

Evacuated tube collectors heat to higher temperatures, with some models providing considerably more solar yield per square metre than flat panels. However, they are more expensive and fragile than flat panels. Evacuated heat tubes perform better than flat plate collectors in cold climates because they only rely on the light they receive and not the outside temperature. The high stagnation temperatures can cause antifreeze to break bown, so careful consideration must be used if selecting this type of system in temperate climates.Tubes come in different levels of quality so the different kinds have to be examined as well. High quality units can efficiently absorb diffuse solar radiation present in cloudy conditions and are unaffected by wind. They also have the same performance in similar light conditions summer and winter.

Solar Hot Water Positives and Negatives

Positives

1. They provide a huge energy saving over the course of a year as the ‘free’ solar heat does all the work.
2. They are robust and require little maintenance
3. During warmer weather there is the option of turning off the auxiliary heating (but not the pumps in active systems. Timeclocks help almost carefree management.
4. The technology works well and is proven for more than 50 years

Negatives

1. They are costly and have a moderate pay back period (this does not include government subsidies or rebates which would improve this considerably)
2. They are costly to install and may require plumbing and reinforcing work, especially if retrofitted
3. Care needs to be taken in choosing the correct type for the climate of the particular region
4. If choosing vacuum tube units, care must be taken to select quality products to avoid high maintenance costs in the future. The ‘best’ is the best and the cheapest will probably bring problems

Questions to Ask

1. Where was the unit made? There is some doubt about the quality of cheap Chinese units (which sell in China for 20% of the cost of a local unit)
2. Is the unit under consideration suitable for the climatic conditions? Factors such as frosts may mean a system with antifreeze and a heat exchanger need to be used.
3. If an active (pumping) system is being considered, what is the reduction in energy savings?
4. If retrofitting, is there a need for reinforcing of the roof to be able to support the unit?
5. What is the optimum angle (related to the latitude of the location) and can this be achieved? Will this require an extra structure that can cost $500 more?
6. How often does the unit require servicing (5 years ??) and are parts and servicing skills locally available?
7. Can installer fit a timeclock with manual over-ride, for winter electricity boost?

Key Statistics for a 300 litre Unit

Item Value Comment Electricity Use 7 kWh/day for four cold months =840 kWh per year versus more than 5,000 kWh for all-electric hot water Requires a time clock and manual over-ride to boost hot water for winter bathing times Greenhouse Impact 1 tonne per year versus 6-7 tonnes per year for all electric hot water. Ballpark Price Ballpark Price $3,000-$5000 net of subsidies and installation Should be mandated on all new houses in Indigo Shire

 

 

 

 

 

Prepared by Klaus Baumgartel Beechworth Sustainability, Energy Machines GroupTuesday, November 11, 2008

A heat pump is a device that moves heat from a source location to a sink location. Most heat pump technologies move heat from low temperature heat source to a higher temperature heat sink. Common examples are refrigerators, freezers, air conditioners an reverse cycle heater cooler combinations.

All matter above absolute zero (appox –273C) contains energy. This can be captured by using the energy present to evaporate a refrigerant (same principle as why evaporating sweat cools you down by taking heat away). This can then be recovered in part by condensing the vapour back to a liquid form by a compressor, the heat being used to heat and the liquid then being recycled through the system.

Because of the compression stage, heat pumps always need electricity to run. They can, however provide a much greater dividend for the power expended than using it for straight heating. In using the heat for hot water a Coefficient of Performance (COP) of between 3 and 4 can be achieved (using an air source system on a mild day) versus a COP of 1 for a straight electric immersion heater i.e. 1 kW of electricity usage gives 3-4 kW of heat in hot water using a heat pump system. COPs tend to get lower as the outside temperature goes down (and the differential to the target temperature increases). However a system developed in Japan has a COP of 3 at an outside temperature of –20C.

COP must not be confused with efficiency. Some sellers will tell you the unit has an efficiency of 300%. This is actually the COP converted to percentages. The technical ‘efficiency’ of the unit itself relates to the refrigerant used, the efficiency of the compressor and the design in relation to expected temperature conditions.

Most heat pumps used by the average consumer will be air-sourced. They will work best when the temperature is higher than when it is colder. The differential between the outside temperature and the target temperature (typically 50-60C for hot water) is what determines the actual COP. The COP drops back to 1 (same as the immersion heater) at –18C. Obviously the heat pump works better in the warmer weather, something it shares with solar hot water.

There are more sophisticated systems that use underground pipes that provide a more stable input temperature but these are expensive and outside the scope of this fact sheet

Positives

1. Significant improvement over existing electric hot water system. This is not as significant if you have gas.
2. Can often plug into existing plumbing with minimal installation modifications required.

Negatives

1. Always needs electricity to run.
2. Are more efficient if the outside air temperature is high than if it is low. At around –18C, the efficiency approaches 1, same as for an immersion heater.
3. Need to ask what the refrigerants chemical is as some are very high impact greenhouse gases in their own right
4. Do require servicing every 3 (???) years

Questions to ask

1. What is the COP of the unit
2. What is the COP at sub zero temperatures (as the region gets in winter)
3. What is the refrigerant used and is it a greenhouse gas in its own right
4. How often does the unit need to be serviced and/or recharged with refrigerant
5. Can installer fit a 24 hour time clock with manual over-ride

Key Statistics for a 300 litre Unit

Item Value Comment Electricity Use 4 kWh/day versus 12-15 kWh per day for a traditional electric immersion hot water heater Can probably be halved by using a timeclock and altering behaviour eg shower when water is hot and do clothes washing in cold water Greenhouse Impact 5 kg of CO2 per day or 1.8 tonnes per year versus 6-7 tonnes per year for all electric hot water The biggest greenhouse impact a household can have is to ditch the old electric hot water. Government subsidy now makes it very cheap. Ballpark Price $1,000-$2000 net of subsidies and installation for a simple replacement A simple one-for-one replacement of an electric hot water can cost less than $700 net of subsidies

Prepared by Klaus Baumgartel Beechworth Sustainability, Energy Machines Group Tuesday, November 11, 2008