Water Heating Solutions
The hot water system (geyser) in most homes is the single largest consumer of electricity and is typically responsible for around 50% of the total electric bill. Clients often ask how they can reduce their electricity bill or even get away from Eskom by going off-grid and the first step is to make the hot water system energy efficient, change to energy efficient lighting and maybe get a gas top stove. Only after these have been taken care of one can look at something like a PV (solar electric) system.
There are a few different technologies that are commonly used to make the hot water system more energy efficient but for most households it will either be a solar water heating or a heat pump water heating system that will work the best and provide the quickest return on investment. For more information on this statement.
PV vs Solar Thermal vs Gas vs Heat Pump Water Heating
The cost of electricity, stability of the national grid and the awareness to sustainable living have forced many consumers to turn to renewable energy technology for heating water. The national building regulations also require that at least 50% of water heating be done with something other than an electrical element. As with many products on the market, one can find a considerable number of contradicting claims with regards to the different technologies. Many of these claims are nothing more than marketing jabber and opinions and have no facts behind them.
A Background to the Technologies
Instantaneous gas water heaters use LP Gas to heat water instantaneously and therefore no geyser (hot water storage tank) is needed. 1kg of LPG “contains” 13.6kWh of energy and therefore can produce 11.5kWh of thermal energy if the gas heater has an average efficiency of 85%.
Solar PV water heating element uses solar electric panels (PV panels) to power an electrical heating element. An electrical element is close to 100% efficient. In other words, if it is a 1kW element it will consume 1kW of electricity to produce 1kW of thermal output. This power now comes from a PV panel. PV panels must be installed facing North and at a horizontal angle equal to the Latitude of the location and it is critical that there is no shading on the panel. Shading as little as 2% of the panel surface can result in more than 50% panel output reduction.
The efficiency of the PV panels is around 17% (under lab conditions with the cell at 25°C). In real life with the sun shining on the cell, it will heat up and cause a decrease in output power of about 8% (0.4%/°C with 20°C NOCT). Panel ageing will typically result in a maximum degradation of 2.5% in the first year and up to 10% in 10 years. Next you need a MPPT to ensure optimal power transfer from the panels to the element. A very good MPPT is typically around 95% efficient and therefore you lose another 5% of the available power.
Solar thermal water heaters use the radiation from the sun to generate heat. The size of the solar panel will determine how much energy can be collected from the sun. If we, for example have a 2.4m2 solar panel connected to a geyser, this might give us 60°C water at the end of a warm sunny day, but during cooler days with less sunshine, it might only be able to heat the water to 35°C. The solar controller will then use Eskom to heat the water to 60°C. If we have a solar panel that is only half the size (1.2m2) we would only get out half the energy and an electrical element will need to do the rest.
A good quality flat plate solar thermal collector has an efficiency of around 73%. The output of a solar thermal collector de-rate based on the temperature difference between the water in the collector and the ambient temperature. Working on an average temperature delta of 17.5°C (heating water from 15°C to 60°C @20°C ambient) will give about a 10% reduction in output. Piping thermal losses on the insulated pipes connecting the solar panel to the geyser is about 10W/m.
Domestic hot water heat pumps use a small amount of electricity to extract a lot of energy from the surrounding air. A heat pump also uses the energy from the sun, although only indirectly, allowing the unit to workday and night, winter, and summer. The efficiency of a heat pump is called COP (coefficient of performance). A COP value of 4 means that the heat pump produces four times the amount of thermal energy as it uses electrically – in other words a 75% saving on the water heating bill.
The COP of a heat pump is dependent on the ambient temperature and the water temperature and so, in a practical domestic hot water system using a well-designed heat pump, a realistic annual COP value is 3 – in other words a 66% saving on the water heating bill.
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Phone: +27 12 661 1920
Mobile: +27 72 341 4104
Thatchfield Hills Estate
VAT No: 4790292157
Our policy is to fully understand and technically interpret our customers’ requirements as well as maintain a relationship through meeting their engineering needs. In addition to the supplying of engineering spares, ConAir Technologies (PTY) Ltd, also offers comprehensive Renewable Energy Solutions, Water Heating Solutions and Cooling Solutions, thereby offering complete solutions to the customers.