Going underground

The completion of the Westway Beacons housing scheme (see BSJ 05/06) represents the first step in the UK for a technology that has the potential to offer compatibility between the potentially conflicting drivers of cost, comfort and low carbon emissions. It is a relatively simple technology known generically as Underground Thermal Energy Storage (UTES).

It is not, however, a new technology – as I found out in the summer at the Ecostock triennial conference on thermal energy storage held at Richard Stockton University in New Jersey. I was surprised to learn that this was the tenth conference, bringing together an international body of experience gathered over 27 years.

Ecostock sounds like three days of music, love and composting toilets, but in fact the only parallel with that festival is that it was a mind-expanding experience. It covered both phase change and UTES, but the programme of UTES events was more than enough to take in.

Papers presented experience and research from all over the world, covering the two major sub-divisions of UTES: ATES (aquifer) and BTES (borehole). For completeness, there is also CTES (cavern), specifically suited to flooded underground caves.

The extraordinary thing about the body of work discussed is the depth of knowledge and shared enthusiasm for a technology that crosses over between geology/ hydrogeology, heat pump design, building heating and cooling design and demand balancing.

Richard Stockton University itself has a 400-borehole BTES system that has been heating and cooling the campus for 11 years and is planning to extend its cooling capacity through a new ATES system for five new buildings. Despite the USA’s generally poor image with respect to energy policy at federal level, there are many other large scale initiatives around the country, often well supported at state or city level.

Among the leading exponents of UTES are Canada, Sweden, Norway, Germany, and Belgium, with a lot of cutting edge research taking place in Japan – but the Netherlands is far ahead of the rest in terms of installed capacity, especially in ATES systems.

My presentation was based on our proposals for an urban heat sharing network for the buildings on the land of the 1851 Commission (South Kensington Museums, Imperial College and Royal Albert Hall) using ATES as a load balancing and interseasonal heat storage mechanism (see report in BSJ 07/06).

UTES applications

The first step in understanding UTES is to differentiate between ground source heating or cooling systems and UTES. There are many applications in the UK of ground water ‘free’ cooling, closed loop ground connected heat pumps, and variations on these themes. The big difference is, not surprisingly, the thermal storage element, which increases efficiency, reduces carbon emissions, and improves return on investment.

The second thing to get clear is the difference between ATES and BTES. The former is on open loop system that draws water from relatively deep aquifers through wells, adds or subtracts heat to it, and returns it to the aquifer at a suitable distance away from the abstraction. The latter involves the construction of underground heat batteries through a borehole field with closed loop tubes grouted in. The closed loop battery is not dissimilar to that used in conjunction with building structural piles, but designed specifically for its thermal energy efficiency.

ATES can only be used where a suitable aquifer exists, and UK hydrogeology is much more variable than that, say, of Holland, which is very uniform. However, aquifers often coincide with cities, and there are large areas of chalk or sand which have the potential to provide the required yields.

ATES has lower capital cost and higher efficiency of underground heat exchange, so our first focus is on this type of system. Where ATES isn’t feasible, BTES almost certainly will be and can still provide a good return for the right load balance.

ATES overview

The key to an ATES system is the reversal of flow to change from heat abstraction to heat rejection. The underground system develops over the first two years in operation into a hot side and a cold side. In winter, the cold side is charged by the extraction of heat at surface level and in summer, the hot side is charged by the addition of heat at surface level. This process of extraction or addition of heat is converted into useful building heating or cooling.

The ideal ATES system is one in which net seasonal heating and cooling demands are in balance – but in the case of an imbalance, heat can be added or subtracted through either summer heat capture or winter heat rejection to atmosphere.

An ATES system falls within the control of the Environment Agency. Fulcrum Consulting is working closely with the EA to establish a regime of licence applications, impact assessments and ongoing abstraction and discharge fees that allow the EA to fulfil its primary obligation to manage and protect the resource while not impeding the technology’s potential where this is compatible.

This is made possible by the fact that the aquifer water is kept completely separate and is returned as extracted, except for a relatively small change in temperature. At the end of each season, the aquifer temperature change is not altered significantly, meeting environmental definitions of sustainability. In Holland, the EA equivalent keeps records of local heat fields generated by ATES systems to ensure that adjacent systems do not interfere. This would also be part of the records built up over time in the UK, along with the standard borehole data.

ATES systems pay back more quickly in terms of cost when there is a significant cooling demand. If there is a greater net cooling demand than that for heating, additional cold charge can be provided by running simple dry coolers in winter. The benefits on plant size and efficiency are immediately apparent.

In Holland, ATES is often the first choice for commercial office buildings on cost grounds, with parallel benefits of reduced plant rooms and lower risk due to mechanical failure. The key for the design allowing the building cooling to be achieved without running the heat pump (aka chiller). The cold store temperature is several degrees lower than the standing aquifer temperature, which makes this significantly easier to do.

The fact that removing heat from buildings becomes a means of capturing renewable energy in summer opens up an interesting series of possibilities. Our typical definition of comfort in residential premises is driven by winter conditions; we are not prepared to tolerate even a small downward adjustment to our preferred 20/21ºC. In summer however we typically tolerate temperatures much higher. This is generally on the basis that the duration is short, and there is a ‘feel good’ factor about warm weather.

In addition, night-time ventilation is often available to allow cooler air in. This could change if warm periods become more extended or extreme, or if muggy nights become commonplace. Our experience is that developers are beginning to look at the added value of cooling, and if this can be delivered within the context of a low carbon technology, it becomes more attractive.

The application of ATES leads to improvements in coefficient of performance (CoP), with resulting savings in CO2 emissions. While the water circulation requires pumps, the raised temperature of the warm store in winter leads to an increase in the seasonal CoP of the heat pump in heating mode, giving a positive CO2 balance. In summer, effective CoP can be based on the circulation pumps alone for most of the load conditions.

Where a development is residential-led with no identified cooling requirement, the engineering logic leads us to look for other buildings with a net excess of heat, such as commercial premises. This leads us towards the concept of an urban heat-sharing network, where net cooling and heating demands of different buildings can be summated and balanced, with temporal offset through the use of the underground store.

From here, it is a short step to the use of UTES systems to address the question of existing buildings, especially historic buildings where overcladding or major alterations to the envelope are not an option. New commercial buildings can reject their excess heat through the heat losses of old buildings in winter.

UTES in the UK

The planning requirements for renewable energy contribution in the UK is driving all developers to look at the beauty parade of listed technologies, and its not long before the options begin to run out. There seems to be an initial rush for biomass, given that it looks like the cheapest bet for compliance, but if it is taken up at anything like the scale that is being considered, questions arise over fuel cost. Maybe this doesn’t matter for developers, because they will provide a 100% gas back-up and current legislation wouldn’t require the users to actually run the biomass boiler, but that’s not going to do much for future carbon emissions, and in any case will surely not prevail as an acceptable provision.

For heating only housing developments, the heat pump CoP is less convincing, and the cost of adding heat to the store, for example by over-sizing solar thermal collectors that also meet domestic hot water demand, was shown by one of the Ecostock presentations to be too high for normal commercial applications.

For heat/cool balanced or net cooling demands however, or where urban heat sharing networks can be created, UTES looks like a promising option, especially where hydrogeology suits an open system. The main barriers at present are the cost of a test bore, essential before 100% feasibility can be determined, the relatively small number of drillers in the UK market that understand ATES requirements, the perceived risk of problems with the EA licensing, and the willingness of building services engineers to get involved with a technology that involves quite a lot of careful consideration in the early design stage. If the knowledge, experience and enthusiasm of the Ecostock delegates can be brought to the commercial UK marketplace, the technology has the potential to deliver a new trilogy of benefits; cost, carbon and community.