Should we be making more use of hot water storage systems?

Published:  16 January, 2018

Isaac Occhipinti and Ian Robinson evaluate the performance and advantages of hot water storage solutions.

Hot water storage solutions in heat networks have been used for decades and have many benefits over systems utilising instantaneous hot water technologies. However, they are still something of an unknown quantity in the UK today. This is a stark contrast to Denmark, where two-thirds of space and water heating is delivered through pipes to buildings in the form of hot water or steam.

Hot water storage solutions have a range of benefits, especially when compared with their counterparts. Systems incorporating a hot water store within each dwelling work on the Flywheel Principle, requiring relatively little energy to keep the whole system topped up.

One such benefit is that they can store the hot water required for certain peak periods of the day, allowing the demand for hot water to be separated from the supply. This means that the hot water can be generated at times outside the peak demand periods of the day and at rates that level the load on the system.

Considering demand

If we consider a typical demand profile of daily hot water usage, we see that typically most of the water is consumed over two three-hour periods, one in the morning and one in the evening.

If demand was constant over the course of the day, this would equate to 4.16% of the daily hot water requirement per hour. This gives the potential to level the load on an hourly basis by recovering the storage over the periods of little demand.

In developments which house over 100 people, the average hot water usage per person per day is a mere 28l at 60°c across the year. In energy terms this average requires 1.63kwh to heat from 10°c to 60°c.

The peak demands are further amplified when we look at the peak instantaneous demand of multiple outlets be-ing used simultaneously within a dwelling. Where a shower and a kitchen sink are running at the same time you can expect a minimum power requirement to generate the hot water of 33kw and larger dwellings could have peaks in excess of 60kw.

These peaks can all be catered for by the storage within the dwellings without having any effect on the demand of the heat network and thus the size of the central plant and the distribution network can be dramatically reduced in size/capacity. Much lower peak primary flow rates (typically below 0.05 litres/second) going to each dwelling have other benefits, such as:

  • Smaller distribution networks are easier, quicker and cheaper to install
  • Lower capital costs of the network which counters the additional cost associated with a cylinder and heat interface unit approach
  • Smaller distribution pumps can be specified which require less energy to run and can turn down to even lower flow rates
  • Greatly reduced pipe sizes
  • Smaller network volumes require smaller expansion vessels and less inhibitor
  • Heat loss in primary circuit is reduced even at higher primary flow temperatures
  • Lower operational cost mean price per unit of energy is lower.

In terms of peak loads on the primary system, using stored water solutions also provides the designer with a great deal of flexibility.

Hot water can be heated through an internal coil in the cylinder or through an external heat exchanger, giving primary return temperatures below 30°c right through the heat up cycle. It offers the use of an electrical immersion heater to provide standby, supplementary or low tariff heat. This allows networks to be switched off altogether during summer months, thus increasing the life expectancy of the central plant.

Hot water can be heated up using a set-back regime where for long periods of the day the primary system can be run at low flow temperatures and then boosted just prior to a peak hot water demand period to maximise the storage capacity.

Delta T control can provide the intelligence needed for a fully automatic and smart heat network where flow temperatures are compensated.

Other recovery regimes include phased charging systems where the network can be split into smaller zones which come on and offline, providing the ability to purge heat from individual heat network branches when heat is no longer required.

This leads onto another key benefit of using storage based solutions – there’s no need to keep the whole network up to temperature using primary bypasses as you would with instantaneous solutions.

Unlike instantaneous hot water solutions, if the heat takes a minute or two to come through the primary system it really doesn’t matter when using a store. Removing the need to keep the primary flow pipes hot has a dramatic effect on the overall distribution losses and operational costs of the network.

Isaac Occhipinti is Head of External Affairs at the Hot Water Association, and Ian Robinson is Technical Manager, Special Applications at Heatrae Sadia

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