Domestic Hot Water production
Domestic Hot water Specified quality (local regulations and standards): drinking water How hot should the water be? Depends on needs and consumption habits Generally: 40-55 C (outflow!), but e.g. in kindergartens 35 C Avoid accidental scald! Cold water temperature: 5-10 C (winter) 20-30 C (summer)
Domestic Hot water Periodic consumption: (industrial areas -interchange) Continuous operation, variable consumption (dwellings, hospitals, office) Continuous operation, given consumption (swimming pool systems)
DHW Demand Daily service water consumption This is often based on the daily consumption of one person ( unit consumption per person ):. q h The value of daily consumption per person multiplied by the number of people (n) will give the degree of daily consumption.. V = Z. q h 24 n Z the ratio of peak consumption and average daily consumption DHW = 40% * V [m 3 /day]
DHW systems Energy source Fossil or solid fluels (natural gas!) Electricity District heating Thermal energy (thermal water) Solar electricity Heat pump General design Local system: one device one outlet Central system: one device group of outlets Type of the device Instantaneous water heater (tankless) Storage water heaters (tank-type) Directly heated (boiler) Indirectly heated (Heat exchanger and a tank: connected serial, parallel or mixed)
Instantaneous water heater Instantaneous water heater (also known as 'tankless', continuous flow'' and 'on-demand' hot water systems) are small wall-mounted units that heat water as you use it, rather than heating it slowly and storing it as a hot water storage tank does. Advantages Continuous hot water flow. System only in use when hot tap is turned on, thus saving fuel when compared to a storage system No tank means low maintenance Disadvantages Delay before the water reaches sufficient temperature Water wastage is generally higher with these systems
Storage tank water heater A hot water storage tank is a well insulated tank (or boiler) designed to store hot water. Most domestic hot water storage tanks also do the job of heating the water they contain, although in some systems (such as dedicated solar setups) the tanks are used only to store hot water. Advantages Storage tank systems provide hot water with minimum ramp-up time The temperature is consistently maintained while the hot water lasts Disadvantages Once out of hot water, it can take some time to reheat the next batch of water. Mild steel tanks require maintenance at least once every 5 years to change the sacrificial anode Hot water tanks occupy a considerable amount of space
Purposes of water storage Provide for an interruption of supply Accommodate peak demand Provide a pressure (head) for gravity supplies Design factors Type and number of fittings Frequency and pattern of use Likelihood and frequency of breakdown of supply (often design for 12- or 24-hour reserve capacity
Indirectly heated water tank connection
Operation of a serial storage tank Total volume flow of the consumption passes through the heat producer During peak loads the performance of the heat producer is insufficient The cooler water that leaves the heat exchanger enters the storage tank where complex fluid dynamics and heat transfer occur The consumer receives the same water temperature that has developed in the storage tank In the tank only those water temperatures can be present that are adequate for the consumer The tank can be regenerated outside the hours of peak load circulation network is inevitable in practice (boiler can be used in smaller systems, where the circulation network wouldn t be created)
Parallel connection Consumer Constant mass flow Variable mass flow
Parallel connection 1. DHW supply system Heat exchanger DHW tank The pressure drop on the DHW producing system is low The role of the pump is the cover the pressure drop of the heat exchanger side The operating point of the pump can be adjusted by a balancing valve The volume flow rate is approx. constant If the consumption equals to the volume flow rate of the pump, there is no flow in the tank; in the case of greater consumption the tank is filled, if the consumption is lower the tank is discharged In order to avoid the overfilling of the tank, it is recommended to turn on/off the pump
Parallel connection 2. Circulation network DHW supply system Heat exchanger DHW tank One pump fills the tank and circulate the water For the time of the peak load period the circulation stops; this is a risk for greater circuits The pump cannot be turned off because of the circulation. For this reason the filling time of the tank is extended outside the peak period The volume flow rate of the filling is low balancing problems may occur! The overfilling of the tank cannot be avoided
Parallel connection 3. DHW supply system The circulation circuit is not loaded by the resistance on the balancing valve that fills the tank Circulation network Heat exchanger DHW tank The circulation volume flow rate is increased; some circulation problems can be solved The circulation does not necessarily stops during peak load The tank can be balanced only in an experimental way
With two pumps The tank filling and the circulation can be independent Circulation network DHW supply system Heat exchanger DHW tank The overfilling of the tank can be avoided Only the volume above the second thermometer can be considered during the sizing procedure It is practical to create a DHW production favouring connection Significant freedom in the selection of equipment sizes
Frequent error: Unbalanced circulation system The distribution of the circulation mass flow is not adequate Supply problems at distant consumers Balancing cannot be avoided by other handmade solutions The outlet of DHW, it s temperature and the increase of circulation mass flow only cover the complaints and uses lots of water and energy
How to choose a hot water system? What fuel types are available? How much water is needed? How far from the hot water system will your taps be? (Circulation!!!!) How much space do you have? What is your initial budget?
Which hot water systems are most efficient? 1. Solar boosted The most efficient systems available, solar boosted systems are generally boosted by either gas or electricity, but get most of their energy from the sun. Bear in mind that if boosted by electricity from non-renewable sources, this may rank the system below natural gas but above heat pumps in terms of operating costs and emissions. 2. Natural gas/lpg Continuous flow gas hot water systems rated at 5 stars or above are the next best option to solar. Tank systems may be a more efficient option for households with four or more people. LPG is roughly on par with natural gas in terms of environmental impact, although it costs more. 3. Heat pump Heat pump technology ranks above electric storage in terms of efficiency, but it still uses electricity and makes a higher contribution to greenhouse gas emissions than gas in most cases. If gas or solar aren't viable where you live, this is the more efficient alternative. Geothermal systems also use similar technology.
4. Electric Which hot water systems are most efficient? Electric tank systems running on electricity from non-renewable sources are inefficient. Each one in Australia puts out, on average, over four tonnes of greenhouse gas per year. These systems are currently the target of a federal phase-out. If you're replacing one of these systems and you don't have solar power, consider other options to see if you could save money and the environment. If you don't have a gas supply and need to use electricity, a heat pump system is likely to be more efficient. Continuous flow electric systems are slightly more efficient too, but they're uncommon because they're expensive to run - and they usually require three phase power too (which also comes with additional costs if you don't already have it).