Heeley City Farm

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Wind & Sun design and install  grid connected thin film PV and wind turbine at a city farm.

Module Type: Unisolar US-64  (10 x 64Wp ); Inverter: 1 x GCI-10000 (0.7kW); Wind Turbine: Proven WT2500

 (2.5kW); Inverter: 1 x SMA WWR-2500 (2.5kW)  

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. 

A new building was designed and built as a training and resource centre by EcoArch which demonstrates a model of contemporary, bio-climatic environmental architecture within an urban context. 

It was conceived as an integrated eco system within the boundaries of the site and aims to be self-reliant in terms of its 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 was reduced to a minimum. Energy design for the building was by LEDA. 

The new buildings and Proven wind turbine are integrated within the existing network of buildings on the site. The south roof hosts an Erisco Bauder living sedum green roof system and the photovoltaic electric panels.

The 0.64kWp Photovoltaic Solar Array 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 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. 

The 2.5kW Wind Turbine is mounted on an 11m self-supporting tower and is sited nearby to the new building on higher ground to 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 grid connected inverters convert DC power generated directly into 230V AC synchronised with the mains grid supply. As the inverters are paralleled with the mains they 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 failure or variation of grid power quality outside of statutory limits. System design was by Wind & Sun.


The wind turbine foundations were laid first and then the wind turbine was installed. Installation of the PV arrays and cabling took a further two days. 

The System in Use

The expected output from the renewables is approximately 3650 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. 

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. In this situation the grid effectively acts as an 'energy store' instead of a site battery system. This avoids 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 grid power failure. However, this was not considered a big issue at the Heeley site. 

Windy Boy & Sunny Boy Inverters

First developed in Germany for use with PV systems, Wind & Sun introduced the first of these inverters to the UK several years ago, see West Wales Eco Centre. Since then ‘G77’ type approval and testing has been developed with and adopted by the UK regional electricity companies to simplify connection procedures for grid connection of  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) adapting this  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 that  small-scale wind power is now a viable option for many more properties. 

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