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There are around 98.6 million units of different kinds of water heaters installed in the world by 2013. Brazil and China lead the demand with 28% each followed by North America and Europe with 11% each followed by Japan (4%), Russia (3%), Saudi Arabia (3%), India (2%) and the rest of the countries account for the remaining 10% (Dawson, 2014). This high share of energy consumption for household heating needs including hot water and respective greenhouse gas (GHG) emissions requires priority attention of the policy makers, investors, manufacturers and building contractors. In addition to greater deployment of SWH and heat pumps replacing the market of less energy efficient water heaters with best available technologies will further decrease environmental emissions and contribute to saving energy. For example, improved efficiency among electric appliances and gas water heaters will reduce CO2 emissions by more than 300 million tons between 2000 and 2020 (Fridley et al., 2007). The following section provides an overview of the potential for energy efficient hot water solutions in different climate regions of the world.
Water is a basic human need. In primitive societies, where the population has traditionally been limited, water used to be extracted from rivers and wells to serve human needs such as bathing, washing clothes and dishes etc. The industrial revolution enabled new techniques for water distribution. This allowed water to become more readily available as pipes became the standard for supplying water to domestic users. With advancing technologies and the demand for better living conditions the demand for heating water rose. Water needs to be heated for human needs like bathing and hand washing especially in winter season and for cooking during the entire year. Different technologies started to develop for heating water for different end-uses including dish washing, laundry, bathing and hand washing which in turn influences energy consumption.
Thermal energy is the main source for domestic and industrial heat around the world (Veerparen & Beerepoot, 2011). The share of residential heat that goes into space and water heating out of total heat ranges from 34 per cent to 39 per cent in OECD (Organization for Economic Cooperation and Development) and BRICS (group of five emerging economies – Brazil, Russia, India, China and South Africa collectively named as BRICS) up to 52 per cent in developing countries. Demand for hot water is significant for bathing, washing clothes and dishes and other end-uses in both developed and developing countries. Hot water consumption depends on uses and application temperature (Mburu, 2009). In Chennai, India for example, following end-use technologies are used for water heating: firewood, animal wastes, liquefied petroleum gas (LPG) stove, geyser, immersion heater and solar water heater (Gopala Krishnan et al., 2003). In Japan hot water use for cooking and bathing accounts for 36 per cent of household energy consumption (Lopes et al., 2004).
A variety of technologies are used for water heating in households around the world. Instantaneous electric water heaters dominated the global water heating market in 2013, commanding highest market share in Brazil followed by electric storage heaters commanding highest shares in China, Europe and North America followed by gas instantaneous water heaters in China, Europe and Japan. Gas storage heaters are popular in North American markets. With around 11 million units of solar thermal water heaters China leads the world market in this segment (Dawson, 2014).
Generally the hot water systems can be classified as conventional (e.g., biomass, gas or electricity based techniques) and non-conventional (e.g., use of solar irradiation or air as heat source). Conventional systems dominate the global water heating market. Electric instantaneous water heaters accounted for highest per cent of the world market for hot water technologies in 2008 (Dawson, 2014). Best available technologies in conventional hot water systems have greater energy efficiency and positive impacts for the environment, economy and health. In warm sunny regions non-conventional solar water heating is cost effective against fossil fuels or electric boilers and their penetration is seen higher in regions with higher solar irradiation like the Middle East and North Africa and southern Europe (on a MW/capita basis). The energy ladder can explain a typical household’s preferred system for hot water. At the bottom of the energy ladder are the households who use conventional biomass fuels like firewood, coal, or animal waste for cooking and water heating. As these households progress on the energy ladder they tend to use more sophisticated energy sources such as LPG, Kerosene gas or electricity. Along with biomass, electricity or gas fired storage and instantaneous water heaters are usually treated as conventional and solar water heaters and heat pumps as non-conventional hot water systems.
Biomass is an energy resource derived from wastes of various human or natural activities. It excludes organic material, which has been transformed into substances such as coal or petroleum through geological processes. Typical examples of biomass are wood (trees, shrubs, wood residue etc.), wastes (municipal solid waste, livestock waste, sewage etc.), crops (starch crops, sugar crops, oil seed crops etc.) and aquatic plants (algae, water weed etc).
As the variety of biomass resources – any renewably available plant-derived organic matter – is distrib-uted widely, the data on heat applications is very uncertain (IEA, 2007). However IPCC raw estimates show that around 35 EJ/year (9.7 PWh/year) of traditional biomass are consumed annually and a further 9.7 EJ/year (2.6 PWh/year) is used for ‘modern’ bioenergy applications (Levine et al., 2007) of which around 3EJ/year (0.8 PWh/year) is thought to be used in industrial and building sector for heat produc-tion purposes that includes the heat component from combined heat and power (CHP) generation (IEA, 2007). According to European Technology Platform on Renewable Heating and Cooling (RHC) biomass currently covers about 92% of the total renewable energy contribution to heating demand in Europe (Alakangas et al., 2014).
These models run on gas or electricity and use a storage tank to store the water that is heated when it enters. When hot water leaves the tank, it is refilled with cold water, which is heated while it is stored.
Gas Storage/Tank Types: In gas storage systems the water stored in the tank is heated by burning either natural gas or LPG delivered via pipes or in bottles. Storage type gas water heaters are all similar in their working. The only variations are in the tank material, the burner and flue technology and whether they are intended for indoor or outdoor installation. The most common tank materials are enamelled steel and stainless steel. Stainless steel tanks usually last longer. Some units have special flue systems to re-circulate the exhaust heat around and outside the tank to increase the heat transfer into the hot water and enhance the tank efficiency.
Electric Storage Type: Conventional electric water heaters have one to two electric elements each with a thermostat. The electric element at the bottom is the standby element, which maintains the minimum thermostat setting while the element at the top sustains hot water delivery when demand is heightened.
There are two types of instantaneous type water heaters: electric and gas instantaneous.
Gas Instantaneous/Tankless Types: Instantaneous hot water systems do not have tanks and only heat the water when it is required. Water is heated by a gas burner when it flows through a coiled pipe called heat exchanger. The burner gets flamed when the water tap is turned on. One type of instantaneous water heater has a fixed burner flame. The temperature of the hot water can be adjusted manually by combining it with cold water supply from the tap. The second type of instantaneous hot water system has electronic controls to adjust the flame size to obtain water at a constant temperature.
Electric Instantaneous or Tankless Type: Tankless water heaters heat water on demand without storing it first in storage tanks. Upon turning on the tap, cold water travels through a pipe into the unit. An electric element heats the water allowing a constant supply of hot water. There is no need to wait for a storage tank to fill up with enough hot water.
Solar hot water systems provide domestic hot water and also contribute to space heating using solar energy. The efficiency of rooftop solar water heaters depends on the type of collector and storage tank among other parameters. Evacuated tube collectors are among the efficient type of solar collectors available capable of delivering hot water temperatures upto 180 °C.
District heating is a system for distributing heat generated at a centralized location for space and water heating. Steam or water is distributed to individual buildings through pipework and, as a result, individual buildings do not need their own water heaters or boilers. Improved energy efficiency is achieved through aggregating a number of diverse consumer loads. District heating systems can consist on a heat only boiler station where a cogeneration plant (also called combined heat and power, CHP) is often additionally added in parallel with the boilers and or geothermal heat.
Heat pumps can provide space heating and cooling and sanitary hot water with a one integrated unit. A heat pump works on the same principle as a refrigerator but instead of throwing out the heat to keep the fridge cool, it pumps the heat into the water. Common forms of heat pumps include:
Water heating is the third largest domestic energy end-use after space heating/cooling and lighting. This could potentially have a significant contribution to global warming. Using efficient water heaters or turning to alternative energy sources can reduce this contribution. Renewable heating systems, for example, solar water heaters are suitable option especially in warm climates.
|Type of Water Heater||Technological innovations||Energy consumption||Minimum Energy Factor*||Minimum First Hour Rating***|
|High-Efficiency Gas Storage||Improved insulation, more effective heat traps, less burner waste, less fuel in the combustion reaction||25.25 GJ/year, 7 per cent less than standard storage models||0.62||254 litres per hour|
|Gas Tank-less||Flow-sensor-activated heating mechanism, improved venting, no standby losses||19.3 GJ/year, 30 per cent less than standard storage models||0.82||9.4 litres per minute at 77° F rise|
|Gas Condensing||Captures more heat from combustion||19.7 GJ/year, almost 30 per cent less than standard storage models||0.8||254 litres per hour|
|Heat Pump||Instead of generating heat, uses electricity to move heat from surrounding air to the water||2,195 kWh/year, 55 per cent less than standard storage models||2.0||189 litres per hour|
|Solar/with gas backup/with electric backup||Uses the sun’s energy to heat water||13.7 GJ/year or 2,429 kWh/year for backup, both 50 per cent less than standard storage models||0.50** Solar
Emissions associated with water heaters can be reduced either through efficiency improvements or the integration of renewable energy (heat pumps, solar thermal) as stand-alone systems (in warm climates) or combination systems. Emissions data from storage systems vs. combination systems is outlined in the chart below.
Today commercial heat pumps save an estimated 120 million tonnes CO2 or 0.5 per cent of global emissions. As outlined in the table below, heat pumps offer a distinct advantage compared to other heating technologies with respect to CO2 emissions (Wemhöner and Afjei, 2005).
|Type||Heat demand (kWh)||Efficiency (%)||Input energy (kWh)||Specific CO2 emissions||Annual CO2 emissions (kg)|
|Electric boiler,||Electric boiler,||Electric boiler,||Electric boiler,||Electric boiler,||Electric boiler,|
|EU electricity mix 2005||15,000||95||15,790||0.472||7,454|
|Electric heat pump, SPF = 3||15,000||300||5,000||0.472||2,360|
|Electric heat pump, SPF = 6||15,000||600||2,500||0.472||1,180|
The following table compares green-house gas emissions (kg CO2/kWh) of different European countries with respect to water heater type. From the table it is clearly evident that emission factor for heat pumps is the lowest (IEA, 2010).
|Emission Factor (Kg CO2/kWh)||Austria||Belgium||France||Germany||Nether-lands||Norway||Sweden||Switzerland||United Kingdom|
|Fuel oil burners||0.2962||0.2962||0.2962||0.2962||0.2962||0.2962||0.2962||0.2962||0.2962|
|Air to water heat pumps||0.078||0.078||0.009||0.173||0.141||0.001||0.006||0.004||0.148|
Energy efficiency of water heaters can be improved by a variety of measure and design improvement options. Adding insulation to storage tank and pipes could result in reducing standing loss up to 45 per cent. Further, up to 60 per cent of waste heat can be recovered by integrating a spiral tube at the bottom of the heat storage tank. Condensing tankless water heaters use 30 per cent less energy than standard storage models. They use exhaust gas to preheat the cold water before it enters to heat exchanger. SWHs can provide for 100 per cent of household hot water needs in warm and tropical regions. Heat pumps are equally suitable for space heating, cooling and water heating in building.
|Technology||avg. energy savings per unit*||Annual Cost Savings (based on 0.12 euro per kWh)|
|Tanks Insulation||592 kWh||71.04|
|Tankless heater||521 kWh||62.52|
|Heat Recovery||up to 1,422 kWh||170.64|
|Heat Pump||2,195 kWh||263.40|
|Solar Thermal||1,185-2370 kWh||142.20-284.40|
For electric storage tank waters heaters a number of design improvements can increase the overall efficiency. Adding insulation, both for pipes and tanks, can reduce standing losses by 25-45 per cent, saving 4-9 per cent in water heating costs. Gas storage tanks can include design features such as electronic ignition, powered exhaust, improved flue baffle and flue damper control (reduces heat loss through the flue vent), condensing heat exchangers and state of the art burners (US DOE, 2009).
Tankless water heaters are 22 per cent more energy efficient than storage versions, which amounts to Eur 54-62/year in savings for the average household. However, this does not necessarily translate into overall cost savings due to higher investment costs of almost twice that of storage water heaters (Euro 617-887 vs. Euro 231-308) and possible issues with temperature inconsistency etc. (Consumerreports.org, 2008; Consumerreports.org, 2015).
In wastewater heat recovery systems, drain water flows through a spiral tube at the bottom of the heat storage tank. Water heater intake water is preheated by circulation through a coil at the top of the tank. This warms the tank water, which rises to the top. Non-storage systems go directly to a water heater or fixture, such as a shower. A maximum of 60 per cent of wastewater heat can be recovered. Preheating water helps increase water-heating capacity and costs Euro 231-386 for a typical household with a payback period of 2.5-7 years (Energy.gov, 2016).
Heat pumps provide space heating and cooling, and hot water in buildings. By integrating a heat pump into a water heating system, EnergyStar estimates savings of 2,662 kWh/year or 55 per cent of water heating costs. Heat pumps have an efficiency rating of approximately 2.2, while high efficiency electrical storage are rated 0.95 (US DOE, 2009).
For developing countries and warm climates, solar water heaters are a particularly applicable technology as they provide a simple, reliable, cost effective method of using the Sun's energy to heat water. In warm, sunny climates solar water heating units can provide 100 per cent of the household hot water needs.
Ahmad ur Rehman Hafiz