Maintaining solar-thermal hot-water supply

Published:  03 October, 2013

Vaillant, solar thermal, DHW, renewable energy
Solar thermal that does not stagnate — John Bailey.

John Bailey of Vaillant explores the challenges faced by solar thermal and explains how opting for an alternative solar drainback system opens up the possibilities for commercial applications.

It seems that interest in renewable technologies is finally starting to ramp up. The roll out of Government incentives can share some of the credit, with domestic RHI finally settling on (and escalating) payback tariffs and the UK recently completing its first Green Deal package. These programmes, combined with an increase in general environmental awareness, are combining to boost the uptake of energy-efficient technologies in both residential and commercial contexts.

Solar technologies in particular have come a long way and, thanks to the revised domestic RHI tariffs being very much in their favour, look to feature much more heavily on UK roofs in the near future. However, what works for one property might not be as suitable for another, particularly when considering commercial applications. One of the challenges for the renewables industry is therefore how to adapt technologies to ensure that there is a choice of effective systems available, whatever the application.

The solar-thermal market currently features two types of technology: pressurised systems, and drainback systems.

A pressurised system is constantly filled with solar fluid in the panels and is heated up by the Sun all the time.

A drainback system is only partially full of solar fluid, which is only sent to the panels to heat when the system requires it.

Stagnation prevented — solar-thermal systems with drainback work on demand and never stagnate.

In the UK, pressurised heating systems currently outnumber solar drainback. Pressurised is a more widespread technology and much more commonly used. However, its prevalence doesn’t necessarily indicate that it is always the most appropriate solar thermal system for commercial buildings. This is because pressurised systems are predisposed to stagnate, which ultimately forces the system to shut down.

Stagnation occurs in pressurised systems because the fluid inside the solar panels continues to heat, raising temperatures and causing the system to expand. This eventually means the system stops running, so it stops providing hot water to the cylinder and releases all the excess fluid into an expansion vessel. A stagnated system cannot start up again and provide hot water until the following day, when the system will have had the chance to cool.

This isn’t usually an issue for residential buildings, as most of the hot water in a household is typically needed in the evenings. However, in commercial buildings such as offices and schools where the hot water demand is larger but subject to fluctuation, it can be problematic.

Solar drainback is a viable alternative to pressurised systems in these scenarios. It never stagnates, because it works like an ‘on-demand’ system. The solar fluid sits within a vessel and is pumped up to the solar collector once the cylinder calls for heat. The fluid is then brought back down to the cylinder to heat the water. As soon as the heat demand in the cylinder is met, the pump stops and the fluid drains from the collector back into the discharge vessel. The system is effectively on call to respond to the weather and the demands of the cylinder, so the fluid doesn’t sit — and then stagnate — in the collector.

Solar drainback for commercial solar-thermal systems offers an alternative renewable solution to consider

Therefore, solar drainback is purposefully designed with commercial applications in mind — buildings where hot-water demand is either sporadic or fluctuates. For example, it’s also beneficial for use in certain properties which can remain empty for a period of time, such as social housing or schools. With pressurised systems, you’d have to drain the fluid out prior to the vacated period; with solar drainback, the system automatically takes care of that for you.

Aside from ensuring that demand can be met at any time of day, the ‘on-demand’ nature of a drainback system ensures that solar fluid lasts a lot longer —increasing the life of a system and reducing on-going costs. In fact, the system is very low maintenance, only requiring annual checks for leaks and to ensure the pumps are still free.

A drainback system is also simpler to install and much more compact than a pressurised system, with a module unit approximately the size of a standard boiler and the absence of an expansion vessel. Vaillant’s auroFLOW system, for example, contains solar collectors, a module (which contains the pumps, discharge vessel, and control), solar fluid and appropriate mounting hardware for in-roof, on-roof, or flat-roof installation.

As a final point, oversizing — which can also be an issue with pressurised systems — is not a concern with solar drainback.

A pressurised system with surplus solar panels leads the system to stagnate more quickly. However, the only effect of a solar drainback system being oversized is that the cylinder heats up more quickly. This does not negate the requirements for proper sizing calculations, but does permit far greater flexibility in terms of array sizes for buildings with irregular demand.

Despite its benefits in certain contexts, the arrival of solar drainback in the commercial sphere doesn’t necessarily indicate a radical near-future shift in the market. Pressurised systems are still efficient and practical technologies, and suitable for a wide range of applications. However, the key point is that solar drainback offers customers an alternative renewable solution to consider, which may well work more effectively for a particular property with a different demand.

John Bailey is commercial systems director with Vaillant.

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