A hot water storage tank (also called a hot water tank, thermal storage tank, hot water thermal storage unit, heat storage tank, hot water cylinder, and geyser) is a water tank used for storing hot water for space heating or domestic use.
Water is a convenient heat storage medium because it has a high specific heat capacity. This means, compared to other substances, it can store more heat per unit of weight. Water is non-toxic and low cost.
An efficiently insulated tank can retain stored heat for days, reducing fuel costs. [1] Hot water tanks may have a built-in gas or oil burner system, electric immersion heaters. Some types use an external heat exchanger such as a central heating system, or heated water from another energy source. The most typical, in the domestic context, is a fossil-fuel burner, electric immersion elements, or a district heating scheme. [2]
Water heaters for washing, bathing, or laundry have thermostat controls to regulate the temperature, in the range of 40 to 60 °C (104 to 140 °F), and are connected to the domestic cold water supply.
Where the local water supply has a high content of dissolved minerals such as limestone, heating the water causes the minerals to precipitate in the tank (scaling). A tank may develop leaks due to corrosion after only a few years, a problem exacerbated by dissolved oxygen in the water which accelerates corrosion of both tank and fittings.
Typically hot water storage tanks are wrapped in heat insulation to reduce energy consumption, speed up the heating process, and maintain the desired operating temperature. Thicker thermal insulation reduces standby heat loss. Water heaters are available with various insulation ratings but it is possible to add layers of extra insulation on the outside of a water heater to reduce heat loss. In extreme conditions, the heater itself might be wholly enclosed in a specially constructed insulated space.
The most commonly available type of water heater insulation is fiberglass, fixed in place with tape or straps or the outer jacket of the water heater. Insulation must not block air flow or combustion gas outflow, where a burner is used.
Another common insulation material used for water storage tanks is polyurethane foam (PUF) insulation. [ citation needed ] Where access to the inner tank is a priority (in cases of particularly aggressive minerals or oxygen levels in the local water supply) the PUF can be applied in encapsulated form, allowing the removal of insulation layer for regular integrity checks and if required, repairs to the water tank.
Very recently sensible storage systems has been commercialized using innovative supported vacuum insulation. This technology is now making it possible to store thermal energy is small to medium-sized systems for weeks without any significant heat losses. Where possible it has the potential to use affordable sensible thermal storage for medium-term energy storage.
In a solar water heating system, a solar hot water storage tank stores heat from solar thermal collectors. [3] The tank has a built-in heat-exchanger to heat domestic cold water. In relatively mild climates, such as the Mediterranean, the (heavily insulated but metal-wrapped) storage tanks are often roof-mounted. All such tanks share the same problems as artificially-heated tanks including limestone deposit and corrosion, and suffer similar reductions in overall efficiency unless scrupulously maintained.
Water heater tanks may be made of vitreous enamel-lined carbon steel, stainless steel, or copper.
While copper and stainless steel domestic hot water tanks are more commonplace in Europe, carbon steel tanks are more common in the United States, where typically the periodic check is neglected, the tank develops a leak whereupon the entire appliance is replaced. [4] Even when neglected, carbon steel tanks tend to last for a few years more than their manufacturer's warranty, which is typically 3 to 12 years in the US.[ citation needed ]
Vitreous-lined tanks are much lower in initial cost, and often include one or more sacrificial anode rods designed to protect the tank from perforation caused by corrosion [5] made necessary since chlorinated water is very corrosive to carbon steel. As it is very nearly impossible to apply any protective coating perfectly (without microscopic cracks or pinhole defects in the protective layer) [6] manufacturers may recommend a periodic check of any sacrificial anode, replacing it when necessary.
Some manufacturers offer an extended warranty kit that includes a replacement anode rod. Because conventional hot water storage tanks can be expected to leak every 5 to 15 years, high-quality installations will include, and most US building/plumbing codes now require, a shallow metal or plastic pan to collect the seepage when it occurs.
This method stores heat in a tank by using external heat-exchangers (coils) that can be directly tapped or used to power other (external) heat-exchangers.
The chief benefit is that by avoiding drawing-off domestic hot water directly, the tank is not continually fed with cold water, which in 'hard' water areas reduces the deposit of limescale to whatever is dissolved in the original charge of water plus relatively trivial amounts added to replace losses due to seepage.
An added benefit is reduced oxygen levels in such a closed system, which allows for some relaxation in the requirements for materials used in the hot water storage tank and the closed water circuits, external heat exchangers, and associated pipework.
While an external heat exchanger system used for domestic hot water will have mineral deposits, descaling agents extend the life of such a system.
Another method to store heat in a hot water storage tank has many names: Stratified hot water storage tank with closed water circuit, stratified thermal storage, thermocline tank and water stratified tank storage but in all cases the significant difference is that pains are taken to maintain the vertical stratification of the water column, in other words to keep the hot water at the top of the tank while the water at the bottom is at a distinctly lower temperature.
This is desirable in places with a wide climatic range where summer cooling is as important as heating in winter, and entails one or more of the following measures:
When a stratified hot water storage tank has closed water circuits, the water temperatures can be up to 90 to 95 °C at the top and 20 to 40 °C at the bottom. Calm, undisturbed water is a relatively poor heat conductor when compared to glass, bricks and soil.
(Illustrated by a still lake, where the surface water can be comfortably warm for swimming but deeper layers be so cold as to represent a danger to swimmers, the same effect as gives rise to notices in London's city docks warning 'Danger Cold Deep Water).
Accordingly, an arbitrary volume of hot water can be stored, as long as the stratification is kept intact. In this case there must not be vertical metal plates or tubes as they would conduct heat through the water layers, defeating the purpose of stratification. When effectively employed this technique can maintain water as high as 95 °C (i.e. just below boiling) yielding a higher energy density, and this energy can be stored a long time provided the hot water remains undiluted.
Depending on the purpose of the installations, water exchanges tapping different levels allow water temperatures appropriate to the required use to be selected. [8]
In many solar heating systems the energy parameters can be read as a function of time, from the 'dwell' time necessary to transform daylight into heat, at its peak the maximum hot water temperature near the top of the tank. [1]
When flow starts from the uppermost outlet, cold water enters the tank at the bottom. This drop in temperature causes the thermostat to switch on the electric heating element at the bottom of the tank. When the water at the top of the tank is drawn off the hot water at the top is displaced by relatively cooler water, the top thermostat turns the top element on. When the flow stops, the elements stay on until their settings are met. [9]
While it is common to have the top and bottom thermostats set differently in order to save energy, the fact that hot water rises means the thermostat controlling the upper element should feed the hottest supply, while the lower element the warmest.
If the thermostats in such a system are reversed - warm feed from the top, hot from the center - it may not only affect the energy efficiency of the system, feeding scalding water to a domestic hot water outlet may be dangerous, or if directed to warm-feed washers damage them beyond repair.
Hot water can cause painful, dangerous scalding injuries, especially in children and the elderly. Water at the outlet should not exceed 49 degrees Celsius. Some jurisdictions set a limit of 49 degrees on tank setpoint temperature. On the other hand, water stored below 60 degrees Celsius can permit the growth of bacteria, such as those that cause Legionnaire's disease, which is a particular danger to those with compromised immune systems. One technical solution would be use of mixing valves at outlets used for sinks, baths or showers, that would automatically mix cold water to maintain a maximum below 49 C. A proposal to add this to the building code of Canada was unsuccessful. [10]
An autonomous building is a building designed to be operated independently from infrastructural support services such as the electric power grid, gas grid, municipal water systems, sewage treatment systems, storm drains, communication services, and in some cases, public roads.
In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
Solar water heating (SWH) is heating water by sunlight, using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.
A solar thermal collector collects heat by absorbing sunlight. The term "solar collector" commonly refers to a device for solar hot water heating, but may refer to large power generating installations such as solar parabolic troughs and solar towers or non-water heating devices such as solar cookers or solar air heaters.
A storage heater or heat bank (Australia) is an electrical heater which stores thermal energy during the evening, or at night when electricity is available at lower cost, and releases the heat during the day as required. Alternatively, solar storage heaters are designed to store solar energy as heat, to be released during the night or other periods where it is required, often making it more cost effective than selling surplus electricity to the grid and buying it back at night.
Thermal energy storage (TES) is the storage of thermal energy for later reuse. Employing widely different technologies, it allows surplus thermal energy to be stored for hours, days, or months. Scale both of storage and use vary from small to large – from individual processes to district, town, or region. Usage examples are the balancing of energy demand between daytime and nighttime, storing summer heat for winter heating, or winter cold for summer cooling. Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials.
Electric heating is a process in which electrical energy is converted directly to heat energy. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts an electric current into heat. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element; the heating element, depicted on the right, uses nichrome wire supported by ceramic insulators.
A circulator pump or circulating pump is a specific type of pump used to circulate gases, liquids, or slurries in a closed circuit with small elevation changes. They are commonly found circulating water in a hydronic heating or cooling system. They are specialized in providing a large flow rate rather than providing much head, as they are supposed to only overcome the friction of a piping system, as opposed to a regular centrifugal pump which may need to lift a fluid significantly.
Renewable heat is an application of renewable energy referring to the generation of heat from renewable sources; for example, feeding radiators with water warmed by focused solar radiation rather than by a fossil fuel boiler. Renewable heat technologies include renewable biofuels, solar heating, geothermal heating, heat pumps and heat exchangers. Insulation is almost always an important factor in how renewable heating is implemented.
Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever needed, such as in the opposing season. For example, heat from solar collectors or waste heat from air conditioning equipment can be gathered in hot months for space heating use when needed, including during winter months. Waste heat from industrial process can similarly be stored and be used much later or the natural cold of winter air can be stored for summertime air conditioning.
A solar combisystem provides both solar space heating and cooling as well as hot water from a common array of solar thermal collectors, usually backed up by an auxiliary non-solar heat source.
The Drake Landing Solar Community (DLSC) is a planned community in Okotoks, Alberta, Canada, equipped with a central solar heating system and other energy efficient technologies. This heating system is the first of its kind in North America, although much larger systems have been built in northern Europe. The 52 homes in the community are heated with a solar district heating system that is charged with heat originating from solar collectors on the garage roofs and is enabled for year-round heating by underground seasonal thermal energy storage (STES).
An instant hot water dispenser or boiling water tap is an appliance that dispenses water at about 94 °C (201 °F) (near-boiling). There are hot-only and hot and cool water models, and the water may be filtered as well as heated. Instant hot water dispensers became popular in the 1970s. Instant hot water dispensers are very similar to portable shower devices; the latter is fitted with a heating element and quickly heats up water, once a switch has been activated.
Thermal destratification is the process of mixing the internal air in a building to eliminate stratified layers and achieve temperature equalization throughout the building envelope.
In spacecraft design, the function of the thermal control system (TCS) is to keep all the spacecraft's component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to the extreme coldness found in the shadows of deep space or to the intense heat found in the unfiltered direct sunlight of outer space. A TCS must also moderate the internal heat generated by the operation of the spacecraft it serves.
Tankless water heaters — also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters — are water heaters that instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil unless the unit is equipped with an internal buffer tank. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication. However, copper heat exchangers are more susceptible to scale buildup than stainless steel heat exchangers.
A storage water heater, or a hot water system (HWS), is a domestic water heating appliance that uses a hot water storage tank to maximize water heating capacity and provide instantaneous delivery of hot water. Conventional storage water heaters use a variety of fuels, including natural gas, propane, fuel oil, and electricity. Less conventional water heating technologies, such as heat pump water heaters and solar water heaters, can also be categorized as storage water heaters.
The Glossary of Geothermal Heating and Cooling provides definitions of many terms used within the Geothermal heat pump industry. The terms in this glossary may be used by industry professionals, for education materials, and by the general public.