Sheffield Heeley City Farm sustainable building with passive solar design

Heeley City Farm is a community based and led training, employment and youth project employing over 30 people, mostly previously unemployed, on a range of environmentally based enterprises. The farm runs a community cafe and a peat-free garden centre on its 2-hectare reclaimed urban brown field site. Other community-based activities include environmental education, organic gardening, recycling, community composting, energy advice, and food and health projects. Well known as a family-friendly visitor attraction with its collection of rare or unusual domestic animal breeds and play facilities, the farm attracts an estimated 100,000 visitors each year. It hosts the largest community festival in the region and one of the city’s most popular play schemes.

Building a Training and Resource Centre was always an ambitious project for the farm and was begun in 1995 after a tragic arson attack destroyed stables and offices and resulted in the death of two much loved horses and a donkey.  Locally raised money was used to lever out support from English Partnerships (now Yorkshire Forward in this region), the European Regional Development Fund and Single Regeneration Budget amongst others.

From a derelict site in 1981 to being one of the largest local employers, the farm continues to develop its aim of bringing economic prosperity to areas and to people who have consistently been left behind by mainstream economic development.  The new facilities are an important symbol of what the farm is trying to do for its community and an affirmation that community economic regeneration demands the highest standards of quality and investment in the poorest sectors of society. The farm has worked hard to move on from high grant dependency to a more entrepreneurial approach to meeting social need.  It always intended that creating environmentally conscious buildings on its site would raise its profile and help establish it as a regional centre for environmental action.  Environmental and financial sustainability can come together if our commitment to meeting the needs of deprived communities and excluded individuals is balanced with a sound business model which recognises the value of seeing green.

The building demonstrates a model of contemporary, bio-climatic environmental architecture within an urban brown field context. It is conceived as an integrated eco system within the boundaries of the site and aims to be self-reliant in terms of using renewable energy service supply systems. The building is a low energy, high thermal mass, passive solar design powered by renewable energy from photovoltaics and a wind turbine. The need for space heating is reduced to a minimum. The building forms an integral part of the disused brown field site of demolished terraced houses. With its living green sedum roof, it is a focal point for the inner city land reclamation project to create an urban city farm for the benefit of the whole community.

The new buildings and Proven wind turbine are integrated within the existing network of buildings on site. The roof hosts an Erisco Bauder living sedum green roof system and the photovoltaic electric panels. The building design was conceived as a hybrid, with heavy weight masonry mass walls with a 250 mm wide cavity super insulated with Dritherm. The 300 mm deep super insulated sedum roof is formed with Fillcrete Masonite I beams spanning the main structure. The I members comprise of solid 47 x 47 mm timber flanges and 8 mm hardboard webs. This makes more use of a tree than does log conversion for solid timber (using 65% less raw material than conventional timber) and can utilise forest thinnings and lower grade faster growing species to create the 300 mm depth of roof for the full fill roof insulation.

Solar shading and night time cooling, from the Swedish Windows triple glazed low E triple glazing, is controlled by an extensive eaves overhang and externally mounted Jalouse shutters. A super insulated and tiled concrete floor slab, with dense concrete block walls, provides thermal mass to the buildings, retaining the sun’s latent heat and reducing the flywheel effect of rapid cooling down or overheating associated with more light-weight buildings. Very careful air tight detailing (as illustrated in the enclosed detail drawing) has reduced incidental air infiltration to a minimum around the window and door openings through the super insulated fabric of the wall structure.

The naturally absorbent and breathing masonry construction creates a vapour hydroscopic envelope without the risk of condensation. Natural tiled floors, non volatile solvent, water based paints and natural OS colour stains to exposed timber along with the avoidance of formaldehyde based toxic materials, have lead to a healthy internal air quality.

Underfloor heating has been used for the whole building, as the heavy-weight construction along with the low risk of over-heating from solar gains, meant that the slow response of this type of heating would not create problems. By directly feeding the underfloor heating pipes from the boiler (rather than using the normal mixing valves) a low flow temperature on the condensing gas boiler could be used, which improves boiler efficiency. Solar panels for hot-water production were initially considered, but cost constraints eventually led to abandoning any hot water storage and using a “combination” boiler to provide instant hot water. As hot-water requirements in the building are very low – there are no showers or kitchen – this seemed a reasonable compromise. The heating is split into three separate zones covering the main sections of the building, with simple programmable thermostats providing time and temperature control to each zone.

renewable energy
passivhaus design architects
Heeley City Farm Zero CO2 Training Centre

When working on projects with renewable electricity generation, it is obviously essential to use energy efficient electrical equipment within the building. Light fittings were selected for high light output and low electrical wattage, generally using compact or linear fluorescent lamps with high-frequency ballasts. Good day lighting to the main rooms means that lights will only be needed on dull days and after dusk. Extract fans for toilets are d.c. low wattage models which use less than three watts when running. 

The expected output from the renewables is approximately 3,650 kWh a year (3,200 from the wind turbine and 450 from the photo-voltaic panels). The installed internal lighting load, including the stables, is less than 2 kW, and annual lighting consumption should be less than 2000kWh. Electricity use for heating and ventilation is minimal, so there is potential for the electricity use in the building to be completely met from the renewables, although this will be highly dependent upon the use of office and other electrical equipment by farm staff.

Having worked hard to create a super insulated low energy design, reducing the potential energy demand for the building, we worked with Steve Wade from Wind and Sun, to design and install a novel renewable wind and solar grid connected energy supply system.

The farm site had an existing grid electricity supply, to which the new building and renewables systems are connected. This works without using any on-site energy storage such as large battery banks, which would have added to the cost and complexity of the design.  Power generated is first used by electrical loads in the building and any surplus exported to the grid, when power being generated is less than that being consumed, top-up is available from the grid and imported as normal. All power flows are ‘seamless’ from the point of view of the user. The aim is to achieve numerical energy autonomy for the building over a whole year. In this situation the grid effectively acts as an ‘energy store’ instead of a site battery system, avoiding the problems and expense of managing and disposing of toxic batteries from the site at 5 to 8 yearly intervals. A downside is that the renewables system does not act as a back-up power supply in the event of power cuts, however, this was not considered a big issue of the Heeley site. In order to give a balanced supply throughout the year a combined wind turbine and photovoltaic (PV) solar system was used:

0.64kWp Photovoltaic Solar Array This consists of 10 Unisolar US-64 64Wp photovoltaic modules. These modules were chosen because they do not use glass in their construction and there were concerns about risks of damage from vandalism if more conventional glass-covered PV’s were used.  The modules are mounted on a framework above the roof, the area beneath the modules being left free of turf. Output and interface with the grid is via an SMA 700W Sunny Boy inverter – this could accommodate more modules, but budgetary constraints restricted the PV system to 10 modules.

2.5kW Wind Turbine.  Mounted on an 11m self-supporting tower the turbine is sited nearby to the new building on higher ground so that it will receive good exposure to the wind. It produces power at ~ 250VDC which is fed to a special version SMA 2.5kW ‘Windy Boy’ inverter. The wind turbine has good low wind characteristics whilst being protected against strong winds and should give reasonable yield despite the urban nature of the Sheffield site.

The inverters convert DC power generated directly into 230 VAC. These AC outputs are then paralleled together, synchronised with the grid supply, and can either supply power to the building’s load circuits or export power to the grid as appropriate. Each of the inverters contains protective circuitry to shut down the systems in the event of power cuts or variation of grid power quality outside statutory limits. First developed in Germany for use with PV systems, Wind & Sun introduced the first of these inverters to the UK several years ago. Since then ‘G77’ type approval and testing has been developed with and adopted by UK regional electricity companies to simplify connection procedures for grid connection photovoltaic systems. The ‘Windy Boy’ inverter used here with the wind turbine is a new development by Wind and Sun (based on experiences at the York Eco-Centre also built by Eco Arc) adapting this PV technology for use with wind power. Concerns of the Yorkshire Electricity engineers were addressed by using a commissioning procedure developed by Wind & Sun with YE to allow them to accept the wind turbine connection under the G77 guidelines. This new ‘Windy Boy’ system should mean small-scale wind power is now a viable option for many more properties.

It was particularly important to the client and Eco Arc to construct the innovative eco building with a local community based contractor with a track record in utilising local labour with integrated training employment schemes. It was fitting that ‘Sheffield Rebuild’ should be the builder for HCF’s new centre as it is a not for profit community business, which has undergone dramatic local growth since starting the company in 1997.

”Rebuild is a community business with a commercial outlook trying to turn entrepreneuralism in to real and structural community benefit.   Rebuild links investment into deprived areas with real jobs and training for local people. People should have the chance to participate in their areas’ regeneration, not sit at home watching outsiders ‘do it to them’.

Sheffield Rebuild started operations in 1997 in the renowned Manor area of Sheffield and now employs around 160 people on over £5.5M worth of construction, insulation and hard and soft landscaping contracts. They receive no direct funding and employ and train often quite challenging individuals solely out of revenue from the contracts they perform. The type of work varies enormously, new build; including houses for Housing Associations, much renovation work from Sheffield City Council and Housing Associations, plus contracts from ‘community’ clients such as this project with Heeley City Farm.

New employees can follow a range of suitable training opportunities with the emphasis being on time to develop and extra support, rather than a ‘quick fix’.   Many of Rebuilds first apprentices are now fully skilled with NVQ3 and one is pursuing an undergraduate course in project management. Rebuild achieved ‘Investors in People’ in 1999.

Core to the Rebuilds’ philosophy is a ‘ local labour strategy’, which is now operating, in several different communities across Sheffield. Rebuild has achieved 80% of employees from the local areas. To win the HCF tender from some vigorous competition Rebuild came in on a very tight budget, there then followed over 18 months of wrangling between HCF and their funders. During this time some sub-contractors were asked to re-quote over ten times. All this, together with a difficult reclaimed site and the innovative ecological design has made for some interesting discussions and a steep learning curve for the building team.

Gordon Wordsworth, one of the founders of Rebuild has been associated with HCF for many years. Gordon’s previous company completed various small projects for HCF including the ‘Challenge Anika’ where £100K worth of work was completed in 3.5 days! The new centre took a little longer to build, but probably looks the better for it.

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