How do I size my buffer tank?

This is a very popular question coming in from customers to us here at Mibec. Often the size of the buffer tank required will depend on numerous factors connected to the type of renewable energy source and the project as a whole. In this short article we explore some of these factors, ‘rules of thumb’ that are used and provide a few examples to give you an insight into how we go about sizing tanks. Our technical staff members are very skilled at calculating tank sizes for heating or cooling projects and are more than happy to provide you with any assistance you need in finding the right solution to meet your needs – give us a call today.

Probably the biggest factor in determining the size of buffer tank required is the size and nature of the heat source. If you take the example of a log-fired biomass boiler they tend to provide a ‘hard and fast’ burning of their fuel and whilst they have many strong points, are not ideal for regularly turning off and on. In this case buffer tanks provide an ideal partner as the tank will retain heat for later use with the flexibility of immediate access without the heat source needing to get up to temperature; this can be very useful for maintaining a more constant output. In this log burning application we tend to start our calculations with the ‘rule of thumb’ that 50 litres of water capacity is required per kW of output and refine the details from there. Therefore a 50kW heat source would require a 2,500 litre buffer tank as a starting point.

The ‘rules of thumb’ for other biomass heat sources can be different too. For example if we switched the log burner for a wood pellet burner we often only need to take around 1/3 of the heat source’s output into account for a buffer tank as the pellet burner has far better modulation over off and on. So for example if we had a 100kW heat source the buffer tank size would need a capacity of around 1,650 litres (100kW x 0.33 x 50 litres/kW). However the final tank size in both of these examples above will also depend on the factors like the flow rates in the system, the ΔT and the pressure (both the starting point and what the system gets up to). Different methods again are used for other renewable technologies such as heat pumps or for chilled water applications rather than heating or hot water.

Another method that we often use for sizing is to look at the load demand profile from the buildings and the number of people living on the project as there tends to be peak demand times from the occupants often in the morning and evening. For example if we had a 100kW Combined Heat and Power (CHP) system supplying 20 apartments and say an investigation was carried out on the project that concluded the full capacity was only needed for 15 mins in every hour (25%), then the buffer tank sizing calculations could be based on a 25kW output. Working with a typical ΔT of 40oC, another ‘rule of thumb’ that is often used is that to raise the temperature of 500 litres of water by 40oC needs 25kW heat input, therefore a 500 litre capacity tank would be a good starting point for this example project. If for example the demand was increased to 50kW then a 1,000 litre capacity tank would be the starting point and so on.

At Mibec we are able to offer a wide range of buffer tanks from leading European manufacturers such as Cordivari. Tank sizes range from 100 litres up to 5,000 litres covering a broad range of requirements. If an off-the-shelf solution can’t be found to meet your project needs then we also offer bespoke designed but cost-effective tanks. All of these special Mibec tanks are custom-made to suit the exact customer requirements in terms of sizes and connections on only a 2-3 week lead time.

We offer a full free of charge specification service covering the whole of the UK, designed to support architects, specifiers or contractors. We can give you knowledgeable, yet impartial advice on selecting the right tank to meet your projects needs. Please feel free to email or call our support department on 01782 959170 (local call rate) where one of our trained advisers will be more than happy to help you.