Today, the boiler sector is developing significantly, due to the multiple alternatives that the user has and the mandatory energy saving measures under the Energy Efficiency Regulation of Buildings (KENAK).
Specifically, KENAK requires the use of solar water heaters or DHW heat pumps by at least 60% in all new homes.
By Mr. Apostolos Efthymiadis *
Dual energy boilers of the electric boiler type provide flexibility to the user, in order to utilize alternative low-cost heat sources, such as e.g. natural gas, heat pump, pellets and wood chips that are burned either in thermodynamic stoves or in corresponding biomass boilers.
Boiler types and definitions
The production and storage boilers of domestic hot water (DHW) or heating water include:
* The well-known electric water heaters of a few tens of liters and with only one heating source: the electrical resistance (simple energy).
* The invoked electric boilers with a capacity of a few tens to hundreds of liters, which in addition to electrical resistance have a second source of energy in the form of heating water through an alternator (dual energy).
* Large boilers, with a capacity of a few hundred to thousands of liters, which are used either in large buildings or in central solar installations, and which have the possibility of two or more heat sources with heating water (double or triple energy).
The hot water boilers are cylindrical in shape, with external thermal insulation and are divided into categories.
Evolution and classification of water heaters
With storage of hot water for use
Summarizing the typology of water heaters, traditional electric water heaters are simple electricity water heaters and store hot water.
The so-called electric boilers usually have the same construction characteristics as electric water heaters but also include a second energy source in the form of heating water, which enters the housing through intake / return pipes and is delivered to the stored hot water with the help of a heat exchanger.
The term "boiler" has historically appeared in large hot water production facilities in hotels and hospitals, and was used for hot water production and storage boilers with the help of heating water and an external heat exchanger.
More generally in the market the foreign term "boiler" is used to describe boilers heated by one or more external heat sources in the form of heating water. With the development of solar water heaters, boilers were often referred to as "dual energy boilers", to indicate the use of solar heat and electrical resistance as an auxiliary heat source.
Solar water heaters include a heat exchanger to harness solar heat, which can be a mantle type or a dipped spiral heat exchanger.
Με αποθήκευση νερού θερμάνσεως
With the new technology of amphitheater solar systems (COMBI) for the production of both domestic hot water and space heating water, a new technology of boilers is developed, which no longer store hot water but heating water. The domestic hot water is produced by the passage of tap water through a spiral heat exchanger located inside the stored heating water.
A typical arrangement of such a solar thermal system is given in Figure 5, the boiler of which is double or triple energy, since it utilizes solar heat through an external heat exchanger as well as two or more heat sources through conventional boilers. From this boiler also start the central heating pipes for the heating of the radiators.
Alternative heat sources
The main source of energy in the boilers was and remains today electricity, the total price of which (including VAT) is about 19 cents per kWh (kilowatt hours) in the household tariff. Taking a degree of efficiency of electrical resistance equal to 100%, then the price of water heating is also 19 minutes per kWh of heating.
Alternatively, diesel fuel fired from a boiler to produce domestic hot water currently has a price of 1.25 euros per liter. Taking the calorific value of one liter of oil equal to 10 kWh per liter, then the price of a produced kilowatt hour amounts to 1.25 / 10 = 0.125 euros per kWh, ie 12.5 l / kWh.
Taking an average efficiency of the boiler and distribution pipes equal to 78%, then the price of one kilowatt hour to produce domestic hot water is 12.5 / 0.78 = 16 l / kWh..
This price is relatively close to the price of 19 l / kWh of electric heating and remains high. Therefore, today people are looking for new cheaper sources of heat, such as:
a) Natural gas, the price of which is about 30% lower than diesel, ie amounts to 16 · 70% = 11.2 l / kWh.
b) The heat pump, which has an average efficiency (COP) of about 3.4, ie has a price of produced kilowatt hours of domestic hot water equal to 19 / 3.4 = 5.6 l / kWh. c) Biomass boilers with pellets and wood chips, with a total price of hot water production of about 6-7 l / kWh.
c) The solar water heaters, which produce about 550 kWh / sq.m. collector per year and have an average system purchase price equal to 230 euros per sq.m. collector surface. Taking an interest rate of 0.20 (for a seven-year loan with an interest rate of 9%), the price of the produced kilowatt hour of hot water is 230 · 20% / 550 = 8.3 l / kWh. Therefore, the use of double or triple energy boilers ensures users the lowest possible price of domestic hot water. It is noted that the boiler that adapts to an existing heat pump for cooling / heating has only a small capital burden of 1-2 l / kWh, which results from the supply and installation price. However, this charge increases greatly if a heat pump is supplied together with a boiler, and is in the range of 10 - 15 l / kWh.
* Mr. Apostolos Efthymiadis is Dr. MIT engineer, qualified mechanical engineer electrical engineer NTUA, curator of the Scientific Committee of Mechanical Engineering of TEE, as well as energy consultant of the Panhellenic Federation of Property Owners P.OM.ID.A. / UIPI.
