Great expectations

An old chicken shed in Hertfordshire might not seem the most likely starting point for cutting edge offices, but for wind farm developer Renewable Energy Systems (RES), it made perfect sense. The partners were determined to create new headquarters that measured up to the firm’s renewable energy credentials and found what they were looking for in 2000, when they came across the old Ovaltine Egg Farm in Kings Langley.

RES’ vision meant a redevelopment, involving the radical alteration and extension of two existing 1930s buildings to create modern offices and an exhibition space, plus catering, conference and meeting facilities.

The partners wanted to produce all the firm’s energy needs on site, installing a wind turbine and using the surrounding agricultural land to produce biomass energy crop, and appointed Max Fordham to develop these strategies.

The solution involved adding a combined thermal and pv solar panel system, groundwater cooling and a large thermal heat store to the wind turbine and biomass boiler.

The office is heated and ventilated with a supply air system, rated at 1 litre/s m2. The air handling unit has three heater batteries, one of which was fed from the solar thermal system to preheat the incoming air at its coldest – this means heat can be added to the air at temperatures below room temperature. The air is then warmed via a heater battery fed by LPHW, heated by the biomass boiler and gas-fired condensing boilers. In the summer, the air is cooled by borehole water passing through the cooler battery. At 11 °C, the borehole water is cold enough to dehumidify the air.

The local temperature control is provided by LPHW local heater batteries on the supply air inlets, for heating, and borehole water ceiling mounted convectors for cooling.

In late 2003, the building was completed and the client moved in; the heat store and biomass boiler were completed later, in the summer of 2004 and winter 2005 respectively.

So, how have the five main elements of the zero-emission design – the biomass boiler, solar array, wind turbine, groundwater cooling and underground heat store – worked in practice? As might be expected, some have performed better than others, and work to fine-tune the systems is ongoing – but overall, the project has been a success. Overleaf, we examine each element in more detail. 

Biomass boiler

The building’s heating needs were to be met primarily by a 100 kW biomass boiler fuelled by Miscanthus (Elephant Grass) – chosen because it doesn’t require much in the way of water, fertiliser or pesticides once established. The crop is harvested annually in late winter and stored as bales, which are shredded before being fed into the biomass boiler by a mechanical auger.

How has it performed?

Five hectares of the land surrounding the offices was planted with Miscanthus in 2002, but after three years, the crop had not flourished and the growth was disappointing – the first crop, in 2005, was a just fraction of the 10 tonnes/hectare expected. Consequently, the Talbot biomass boiler was fed with straw and wood chip from local sources and little from the site. From being commissioned in January 2005, the biomass boiler provided 22 MWh of heat out of the building’s total requirement of around 170 MWh.

What’s next?

Another Miscanthus crop will be planted in 2006, with a higher density of rhizome and a greater amount of ground preparation, to see how that fares. The feeding of the biomass through the shredder and into the silo has not proved a convenient method and will need to be improved in the longer term.

Monitoring the building revealed that the heating consumption in the first year was twice that anticipated. The 24-hour plant operation and obvious shortcomings with the building’s fabric – including gaps, uninsulated areas and the complex geometry of the roof structure which made insulating and sealing the fabric difficult – have been dealt with, and consumption in the second year has come down by 20%. Further work in these areas, such as finding areas where the insulation may not be as effective as anticipated, will be required to find additional reductions.

Lessons learned

Biomass production and management needs to be much more industrialised to be suitable for all but farmers and enthusiasts. The task of growing and managing a crop on the site to meet most of the heating requirement for the entire building is an ambitious aim.

Wind turbine

The 29 m-tall Vesta wind turbine was purchased second-hand from a farm in Holland, and has a 225 kW peak output. Around seven years old when it was purchased, it has an anticipated operational life of 20 years.

Any excess electricity produced is exported to the grid – and when the turbine is not generating, electricity is imported to meet the

building’s needs. The noise from the M25 and west coast mainline railway line bordering the site was seen as a benefit in this instance, as it would serve to mask any noise generated by the turbine.

How has it performed?

The wind turbine has generated reliably from the start. It was predicted that it would generate 250 MWh annually, compared to the company’s predicted annual consumption of

115 MWh. Measurements for 2004 revealed that the turbine generated 202 MWh, but the building’s electricity consumption was about twice that anticipated, at 201 kWh. This has been put down to the hours of consumption and the high IT load.

In the first eight months of 2005, the turbine has generated 124 MWh compared to a consumption of 134 MWh – similar figures for the same period in 2004.

The wind turbine (with a small contribution from the PVs) did meet the building’s electrical load, but without the comfortable margin expected. The extensive metering incorporated in the building design enabled the team to find out where the electricity was going and make recommendations for reductions with better control and management of the out-of-hours loads.

Recommendations put forward for tackling this include reducing the hours of operation to those when the building is occupied, linking the circulation lighting to intruder alarms and daylight sensing and ensuring IT equipment is switched off at night. This also explains why the building’s electricity consumption goes down very little at weekends. Another issue has been the peaky production from the wind, with highs of 3000 kWh/day.

Lessons learned

The wind turbine was certainly the most impressive renewable source on the project.

In coastal or windier locations, it would easily be possible to produce two to three times as much electricity as is produced here.

Solar array

The 170 m2 solar array includes 116 m2 of solar thermal panels and 54 m2 of combined thermal and PV panels. The design team received an EU framework 5 grant to develop the latter, which are linked to the seasonal heat store and space heating system. Made in Holland by Zen Solar, they incorporate shell solar polycrystalline PVs laminated onto the panel fronts by the Dutch research organisation ECN, and copper heat exchangers on the reverse side to capture the remaining solar energy.

During the summer, autumn and spring when there is little or no demand for heat in the building, it is accumulated in a heat store and extracted when it is needed in the winter (see page 58 for further details).

How has it performed?

The combined solar thermal and PV panels produced heat that was used directly in the building in the winter of 2003/4. They were predicted to generate 40 MWh/year (235 kWh/m2). In 2004, they generated 33 MWh (194 kWh/m2), while between September 2004 and August 2005, the total collected from the panels was 59 MWh (347 kWh/m2). The inverters on the PV array failed for no apparent reason in the first year – they have since been replaced with units from a different manufacturer – so only data from the second year is valid. The PVs on the panels produced about what was anticipated, at 55 kWh/m2.

Lessons learned

The combined solar thermal and PV panel are currently still expensive and somewhat experimental. It could be argued that separating the two panels would be better because it would reduce the complexity of the devices. However, the combined panels will be useful in tight urban sites, where roof area for solar energy is at a premium, to meet local renewable generation targets. They produce hot water and electricity without too much degradation of performance of either and the heat can be used to provide hot water in dwellings or to drive heat-based cooling systems.