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Integrate to accumulate in solar applications

Effective integration of solar technology in commercial installations can pay real dividends and result in a significantly improved installation, according to Steve Cooper
The growing use of solar technology in domestic installations has increased awareness of renewable on-site generation for the general public. As an industry, however, we should not assume that commercial scale installations are similarly straightforward. Scaling-up solar technology for larger buildings introduces challenges that do not arise in domestic settings.

It isn't a matter of 'getting it to work', or of calculating the theoretical payback time of the solar equipment. It is about optimising the system to ensure the lowest possible consumption for the site throughout the lifetime of the equipment. And it is a sad reality that many of today's system designs end up wasting the energy and carbon savings that have been gained through the incorporation of solar and other renewables. So why is this?

The usual suspect is ineffective integration of the renewable equipment with other low carbon equipment such as condensing boilers and variable speed pumps. Solar technology, perhaps even more so than other renewables, is often treated as a 'bolt on'. This can lead, not just to under-performance of the panels themselves, but to wasted opportunities for carbon reduction from the system as a whole.

When you look at some of the technical and practical challenges of integrating the different zero and low carbon technologies, it is really no surprise. As Figure 1 illustrates, there are considerable differences between the optimum operating temperatures of the different low and zero carbon technologies. Finding a solution which does not force any part of the system to operate at a system temperature which compromises its energy efficiency can be a challenge.

For example, if the system temperature is designed too high, a condensing boiler will not operate in its efficient condensing mode and the potential carbon savings will be lost. In the UK, low temperature hot water heating systems were traditionally designed with a supply temperature of 80 deg C (180 deg F) and a return of 70 deg C (160 deg F) based on the requirements of traditional boiler technology and to tackle problems caused by condensation of the flue gases. Today, however, the majority of commercial heating systems in the UK incorporate gas fired condensing boilers.

These are designed to deliver high efficiency by extracting latent heat from the flue gases. Extracting heat reduces the temperature of the flue gases below the dew point, which for natural gas is around 54 deg C. When flue gases drop below the dew point, condensation forms. This condensate is acidic and quickly corrodes ferrous boiler and flue materials. So, unlike traditional boilers which were built from steel or cast iron, condensing boilers are built from aluminium or stainless steel.

But here's the problem. Over 90 per cent of the system designs we see which incorporate condensing boilers are still operating at 80 deg/70 deg C. They will never condense during normal operation. The situation is exacerbated when you add solar into the mix. So how do you arrive at effective operating temperatures that will harness the considerable energy savings of the condensing boilers at the same time as optimising the solar capabilities?

Pre-engineered solution
A pre-engineered solar heating solution (an approach developed by Armstrong) is shown in Figure 2. This is designed to produce DHWS at 60 deg C with gas fired condensing boiler back-up. It is suitable for DHWS loads up to 1600 litres per day.

A biodegradable propylene glycol solution is circulated through the solar array by the solar pump station once a predetermined temperature differential is established between TS1 and TS2. The solar array heats the water/glycol mixture which, in turn, heats the lower portion of the stainless steel solar cylinder. Where there is insufficient solar energy available to achieve 60 deg C at TS3, the LZC controller automatically brings in the back-up boiler for instant top-up.

The LZC controller ensures that maximum solar energy is used while, at the same time, ensuring that the back-up boiler operates in condensing mode.

Many commercial-scale projects involving solar, however, also incorporate other forms of on-site generation such as biomass and heat pumps. For projects like this our approach is to integrate the renewable devices with a thermal store, combined with an LZC controller, alongside HVAC equipment that is specifically designed for all variable speed operation.

The thermal store typically performs three functions. First, it levels out supply and demand. For example, solar energy available during the day can be stored for release at night. Secondly, it acts as a buffer to prevent boiler cycling and ensure residual heat from a biomass combustion chamber is adequately dissipated. And, finally, a correctly designed thermal store allows us to vertically stratify temperatures.

Hot water rises and cold water sinks, so lower temperatures, from solar thermal or heat pumps, are fed into the bottom of the store. Medium temperatures from condensing boilers are fed into the middle of the store, and higher temperatures from biomass are fed in towards the top of the store.

In order to adjust to greater variability however you need to have the right blend of HVAC components across the system. Traditional HVAC equipment typically cannot provide this flexibility, but that doesn't mean to say that untried, bespoke equipment is the answer.

To summarise, the key to harnessing the considerable potential of renewable technology such as solar is to reject the 'bolt-on' approach and consider the system 'holistically' through effective integration and selection of operating temperatures.

//The author is director - sustainable design at Armstrong //y
14 May 2013

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