How to design a biomass CHP plant? 

Generation of heat, electricity or combined heat and power based on biomass feedstock can be achieved using a range of conversion technologies, including combustion boiler and steam turbine plant, gasifier and IC engine/gas turbine equipment and, for medium temperature heat sources, organic Rankine cycle (ORC) systems. 

The type of technology adopted will depend, to a large extent, on the available biomass resource and its characteristics. Heat and power can be produced from biomass in the form of forest and agricultural residues, demolition wood wastes and a wide range of other organic materials, such as animal slurry, food waste, sewage sludge, SRC, other energy crops and imported materials such as palm kernel expeller (KPE), olive residues, etc. 

Once the fuel supply(ies) has been decided, it is essential to confirm a range of issues, including the security of the supply chain(s) and seasonal variations, costs, fuel storage, rate of consumption, byproduct utilization/disposal, etc., and address the planning/environmental regulatory aspects. 

Gasification processes are likely to be the most technically challenging process route but for all methods of conversion, the energy efficiency, emissions, waste production, etc. will need consideration. The selection of the prime mover is more straight-forward. The popular choice for utilizing biogas from landfill/mine sites, anaerobic digestion or gasification is either IC engines or, less commonly due to lower efficiency, gas turbines. 

The optimum choice of prime mover depends on diverse factors such as heat and electricity demands, seasonal demand variations, thermal efficiency, financial returns required on the investment and risk considerations. Other key factors to consider include the cooling requirements, integration with heat/power demands and connection to the electricity grid or consumer. 

Our consultants have developed design models that can help you to assess the feasibility and likely performance of the selected system and offer advice on any aspect of biomass CHP schemes. 

How much biomass fuel is required for a 10 MWe power plant?

A 10 MWe (80 GWh) biomass power plant will require approximately 50,000 to 75,000 tonne/year of fuel, depending on its type, characteristics and efficiency of the power plant. 

How does anaerobic digestion (AD) work? 

The AD process can be used to treat agricultural, household and industrial residues and sewage sludge. It takes place in a warmed, insulated container (digester), which creates the ideal conditions for bacteria to ferment the organic material in oxygen-free (anaerobic) conditions. The digester must be warmed (about 37ºC – mesophilic or 55ºC – thermophilic) and the raw materials mixed thoroughly to create the ideal digestion medium for the bacteria to convert organic matter into biogas. The biogas is a mixture of methane and carbon dioxide (in a roughly 2:1 ratio), which will have a typical calorific value of 6.5kWh/m3, which is approximately half of the calorific value of natural gas. This gas can be used to generate heat or electricity, while the residual sludge and liquor retains its nitrogen content to yield a good quality soil conditioner and liquid fertiliser. 
 

What is the operating range of wind turbines? 

The energy contained in the wind is proportional to the cube of the wind speed.

This means that a doubling of wind speed would result in an increase in the available energy by a factor of 8. The power curve of a wind turbine shows how much power can be produced from the system at different wind speeds. Wind turbines start operating at wind speeds in the range of about 4 to 5m/s (about 10 miles/hour) and reach maximum power output at around 15 m/s (30 miles/hour) to 25 m/s (50 miles/hour). Above wind speeds around 25 m/s, wind turbines normally shut down automatically due to safety considerations. 

How efficient are wind turbines? 

The amount of energy that can be extracted from the wind has a theoretical limit of about 60% of the theoretical maximum (known as the Betz Limit). Modern wind turbines are close to this limit over the operating range of wind speeds. However, due to the intermittent nature of wind, on average a wind turbine in the UK typically produces over a year roughly 35% of the maximum output. 

How to apply photovoltaic modules in a build environment?

Photovoltaic modules convert sunlight directly into DC electricity and can be integrated into buildings in various ways: on sloped roofs and flat roofs, in façades, atria and shading devices. Modules can be mounted using frames or they can be fully incorporated into the building fabric; for example, PV roof tiles are now available and may be designed as an integral part of the exterior cladding to add a stylish and functional façade.

A typical array on a domestic dwelling would be 9 to 18m2 and would commonly produce between 1 to 2kWe peak output.

For best performance, PV modules need to be at an angle of 20-40 degrees, and facing south. However, this is not always possible on existing buildings. To function well, PV installations need to be inclined at between 10 and 60 degrees, and facing from east to west (i.e. within 90 degrees of due south). Connections between individual panels can be made either in the support structure, or inside the roof void, and are rarely visible from the exterior of the building. Although roof mounted PV is the most common, modules can also be mounted on the sides of buildings or on freestanding support structures on the ground. Other examples of building integrated PV include external sun shading of windows and glass atrium roofs. 

What are ROCs and how much are they worth? 

The UK Government enacted the “Renewable Obligation Order”, which created a system of fiscal incentives for generators to produce electricity from certain “qualifying” renewable resources. This incentive scheme, which is administered by Ofgem, takes the form of “Renewable Obligation Certificates” (ROCs). One MWh of electricity from renewable sources entitles the generator to one ROC. When a renewable generator sells electricity to an electricity supplier it is common, though not necessary, to sell the ROC too. At the end of the year (end of March), the supplier proves to Ofgem that they have met their renewable obligation by submitting the correct number of ROCs. If they fail to do so, they must pay a “buy-out” fine.  

The ROC is also worth an extra bonus because it entitles the supplier to a share of the “buy-out” fines at the end of the year. 

The ROCs auction is managed by the Non-Fossil Purchasing Agency (NFPA). The average price of ROCs was £49.27/MWh in October 2007 auction. (The average price of co-fired ROCs was £49.11/MWh.) 

The energy regulator Ofgem has published the buy-out price for the Renewable Obligation between April 2007 and March 2008 as £34.30. The value of the buy-out fund is recycled to the electricity suppliers in proportion to the ROCs they have submitted. 

At present, the Government is consulting on the future of the ROC scheme and is proposing a “banding” arrangement of ROCs for different technologies from April 2009. 

What is Climate Change Levy (CCL) and how does it work? 

CCL is “a tax” on energy used by businesses that was announced in the March 1999 budget and implemented in April 2001.  

The CCL requires electricity suppliers to charge commercial customers (i.e. business not domestic, governmental or charitable customers) an extra 0.43p per kWh (i.e. £4.30 per MWh). This money is remitted to the government and is used to fund a national insurance contribution break and energy saving programmes.

 
Electricity produced from qualifying “renewable” sources is exempt from CCL, and is qualified for levy exemption certificates (LECs), which can be bundled with the power when sold to a supplier.