Summary of our current “carbon situation”

We believe The Milll is now carbon neutral. Prior to our various renewable technology investments we used about 17,000kWh annually for lighting, water heating and electrical equipment and appliances.  This is equivalent to about 9 tonnes of CO2 emissions per year.  In addition we burned around 5800 litres of oil for heating.  This is equivalent to around 14 tonnes of CO2 emissions.   We now no longer burn any oil for heating.  Much of the electricity we use is self-generated from our hydro-turbine.  Depending on our usage we may be importing electricity (for example when our heat pump is on and drawing around 8kWh), or exporting it (for example during much of the summer when the heat pump is not operating).  Prior to the installation of our heat pump in the course of a year we generated about 2000kWh more than we used.  Since the installation of our heat pump and an all electric kitchen our total electricity usage has jumped.  Currently (2015) our monthly electricity payments are £172 (this is for the mill itself with its six bedrooms and bathrooms and for the holiday cottages with 3 bedrooms and three bathrooms).  This is £2064 per annum.  Against this we receive around £1800 for our renewable feed in tariff plus exported electricty.  All the additional electricity imported is now from a green tariff and entirely sourced from renewable sources. In late 2014 we applied for Renewable Heating Incentive payments reflecting our use of the heat pump to heat the house.

Details of our various renewable initiatives

The micro-hydro-turbine

In 2005 we commissioned Derwent Hydro to undertake a feasibility study.  This indicated a potential generating capacity of around 30,000kWh per year and identified a suitable location.  Derwent Hydro recommended a siphonic turbine with a flow rate of 450 litres per second operating on a head of 1.4 metres and with a potential output (at 80% efficiency) of just under 5 kW.  After attending to the various planning, regulatory and civil engineering issues the turbine went live at the end of November 2006.  We received a grant of £5000 towards the total cost of some £30,000.  At the time of its installation we received payments of around £45 for every MWh produced, plus around 7.6p per every kWh exported.  Recent changes in incentive payments mean that we now receive just over £100 for every MWh produced but only just over 3p per unit for our exports.  For those thinking of doing a similar thing please note that had we installed the turbine after July 15th 2009 we would be receiving over £200 for every MWh produced!

The solar panels

We decided to install solar panels because most of our hot water demand is during the summer months.  We therefore thought this would reduce significantly the amount of electricity used to heat water (which we could offset against the additional electricity imported during the winter to heat the house).  We ordered the solar panels from Smart Energy in 2006 but put off installing them until July 2009 because we were not clear where our hot water tank would be after our renovations.  We were extremely lucky to have them installed when we did because Smart Energy went into liquidation a couple of days after the installation.  Our solar panels cost around £7000.  However, because hot water demand from our guests is first thing in the morning we effectively use the heat pump to heat water, with the solar panels only providing a minor top up.

The heat pump

Our hydro engineers planted the idea of using our self-generated electricity to run a heat pump as it is far more economical to use our self-generated electricity than to export it.  The heat pump is manufactured by Wiessmann in Germany and was installed by local company Renenergy.  It has a rated output of around 27kW for which it requires between 5 and 8 kW of input (plus the occasional boost from a 6kW 3 phase immersion heater in conditions of extreme cold).  The heat pump extracts heat from the river by means of heat exchange coils submerged in the river silt.  The original design pumped water from the river through a self-cleaning filter and thence into a plated heat-exchanger.  This design never worked effectively because it was too difficult to balance the flows between the various elements.  We found that the heat exchanger would regularly freeze solid and then take many hours to defrost whereupon it would immediately freeze solid again if the heat pump restarted.  Renenergy engineers spent many hours trying to ensure the pump primed effectively, replacing pumps, changing pump locations, increasing the bore of the various hoses and eventually considering changing the specification of the heat exchanger.  They finally changed the design from a direct-water sourced system to the indirect system.  Since when the heat pump has been operating much more effectively.

We started to receive Renewable Heat Incentive payments from November 2014 for using our heat pump to heat the house.  Calculations indicate our seasonally adjusted performance factor is 3.7 meaning that for every kW of electricity we use we get 3.7kW of heat.  This is a good performance factor, which together with our heat demand resulted in a payment of  £7743.24 (inflation adjusted each year) per year for the next 7 years.

Our expenditure on the heat pump and associated equipment was around £23,000 so the RHI will more than pay for the capital and installation costs.

The underfloor heating (UFH)

A heat pump works most efficiently when producing output water temperatures of around 45 degrees.  This is not hot enough to run a radiator-based central heating system which uses water at temperatures in excess of 65 degrees.   Most heat pumps therefore operate through underfloor heating schemes which have a much larger surface area and run at a much lower temperature. We have installed underfloor heating on the ground and first floor of the mill.  Our top floor rooms are not heavily used in the winter and can be heated when required by small electric panel radiators.  We used Eco-Conversions (sister company to Renenergy) for the design and installation of the UFH.

The heating is provided by several kilometers of high quality, flexible plastic PEX pipe embedded in screeding on the ground floor and in a dry sand and cement mix on the first floor.  The pipes are connected at three manifolds which control the flow of water into each heating loop via electrically operated valves.  Beneath the pipes on the ground floor there is a significant layer of rigid foam insulation to prevent downward heat loss.

The system comprises 12 independently controlled heating zones with wall mounted thermostats and controls in every zone.  Each zone can be set to a different target temperature and a “set-back” temperature which it will revert to when the heating is “off”.  All zones can have independent switching times.  One benefit of the system is that if we light our log-burning stove in the sitting room, the room thermostat will recognise this and reduce the circulation of hot water to the UFH system in the sitting room thereby saving energy.  Additionally we can set our guest bedrooms to be “off” during the day but on in the morning and evening.

Our expenditure on the UFH was around £9,000

The rainwater recycling system

We discovered a disused coal cellar next to the house when undertaking our renovations.  We cleaned out the rubble and tanked it.  Rainwater is collected from the rear roof and sent to the tank via a filter in the down-pipe.  It is delivered to the tank through a calmed inlet which prevents incoming water from stirring up any sediment.  The tank contains a floating and filtered intake hose attached to a pump which pumps rainwater to a holding tank in the attic.  This then delivers it to all the ensuite toilets.  The system is managed by a computerised panel which detects the level of water in the collection tank (and if it is insufficient delivers mains water to the attic tank).  It also measures the water in the holding tank and only refills it when it has emptied to a certain level.  This stops the pump coming on every time a toilet is flushed.  It also detects if the tank has not been used in the previous 24 hours,  in which case it drains water in the attic tank back into the collection tank where the water remains cool and fresh.  The rainwater management system (Rain Director) was provided by RainwaterHarvesting.co.uk.  Total expenditure has been around £1000.

This system has never worked for us because of leaks in the tanking which means that as soon as water is delivered to the tank it seeps out into the water table.  An attempted repair by the original tanking company has failed to solve the problem so it looks as though we will have to have a flexible waterproof liner specially made.  This will have to be small enough to pass through the manhole cover.  All the electronics seems to work OK, it’s just that there is never sufficient water in the tank to be useful.

Domestic hot water

We use three energy sources to heat our hot water.  The heat pump provides energy efficient heating of hot water to around 45 degrees.  The solar panels will provide solar thermal hot water in the summer and on sunny days in the winter.  Finally we have a 3 phase immersion heater to ensure our water reaches temperatures that will remove any risk of Legionnaire’s disease and to provide back up.  The heat pump is timed to switch to domestic hot water only outside daylight hours to ensure we do not waste any available solar energy.  Similarly the immersion heater is timed to boost temperatures in the evening and during the early hours of the morning.

The solar panels have not been a good investment for us, primarily because our guests require hot water for showers first thing in the morning.  This means we have to ensure the water is hot enough before the sun rises – and in the event that it doesn’t get heavily used the sun then has nothing to heat!

We have a relatively large building with six bedrooms all with ensuite bathrooms.  Many of these rooms are some distance from the hot water tank on the ground floor.  We have therefore installed a pumped circulation to ensure hot water is available quickly at all taps at times of maximum use.  The circulation system is on a timer because pumping water around the house uses electricity, loses heat and cools the water in the hot water tank.

We try to be economical with hot water because of the energy required to heat it.  We therefore do not provide baths and all our showers have water saving shower heads (Mira Ecoflow).  These heads mix air into the water stream and provide a very full shower experience but at a much reduced rate of water consumption.

Other elements

All our toilets have dual, low-volume flush systems to reduce water consumption and loading of our septic tank systems.

Most of our lights are compact fluorescent, low energy bulbs.  We have some LED emergency exit signs, picture lights and external floodlights.  I have recently changed all the halogen downlighters in the cottages to LED GU10 (240 volt), reducing the power consumption from each light from 40W to 5W.  The quality of light from these LED bulbs is good although a bit colder in light quality than the halogens.  Apart from economics the main reason for changing the halogens was my fear of fire.  I have recently discovered charred power supply cords to halogen lights, which I think suggests a considerable fire risk. LED lighting is now (2014) becoming a good option as the quality of light emission has improved and prices have dropped.

We have sought to reduce our exposure to volatile organic compounds (VOC) in the building by using natural, zero VOC paints for all new paintwork and water-based wax treatment for all new wood.  Unfortunately we were unable to source natural, non-slip floor coverings for our wet-room bathrooms and kitchen. These are PVC-based.

We are reducing our reliance on chemical cleaning products by designing easy to clean bathrooms and E-cloths wherever possible.

Our rebuilding work enabled us to radically improve the thermal efficiency of the building by the use of rigid foam in floors and ceilings, spun mineral wool in some voids and for sound-proofing in walls, and sheeps wool insulation in the attic.  Additionally practically all windows are now double-glazed.  The curtain wall at the rear of the house uses 100% recyclable aluminium framing which is powder-coated and virtually maintenance free (the glass is also self-cleaning).  The remaining wood, sliding sash and casement windows use laminated Scandinavian softwood.

We have an Owl wireless energy meter which enables us to measure our consumption of electricity at any time and at any place in the house.  This helps to keep us focussed on minimising our importation of electricity.

We have recently put a micro-sewage treatment plant into our septic tank.  This breaks down solids using aerobic digestion with the air supply being provided by a 50W air pump. The quality of the effluent is good enough for us to be able to gravity feed the outflow direct to the river (with, of course, an appropriate discharge licence from the Environment Agency).  This avoids all the problems we used to have with pumping effluent from a pump inside the septic tank, along a long length of narrow bore pipe to a soakaway.

Solar PV  Solar electricity is about the only renewable technology not demonstrated at the Mill.  However in November 2014 we put in a planning application to replace the roof of our shed and greenhouse to enable us to put a solar array on the South facing roof.  The application requested a raising of the roof line by about 4.5 feet to create a slope of 35 degrees, which is around the optimal elevation for solar PV at this latitude.  We received planning approval for these changes in January 2015.   Delays in reconstructing the roof, together with proposed radical cuts in the tariffs for generating electricity through solar PV has meant that we are now rushing (December 2015) to install a ground mounted 7kW system south of the shed and move this to the new shed roof when it is constructed.  The solar PV will be installed on 8th January 2016, 7 days before the deadline after which generation tariffs would be severely cut and undermine the economic feasibility of the installation.  Although tariffs have come down a lot since the heady days of around 43p per kW, the price of solar panels has also fallen.  We expect to be receiving 10.4 p per kW, against an investment of just over £8000.  With generation tariff payments, sell-back payments and savings from our small but remaining electricity bill, we think we should be able to pay back this investment in 4 -5 years, which is a good return.  Unfortunately it is likely that the radical generation tariff cuts proposed by this ‘greenest government ever” will reduce the attractiveness of solar panel installation, unless solar panel costs continue to come down significantly.

Economic considerations

Our investments in renewable technology were primarily motivated by a concern for the environment and a desire to use our natural resources in the most effective manner, rather than any financial payback.  However with changes in the arrangements for payments for renewable electricity generation and incoming payments under the Renewable Heat Incentive our investments look increasingly attractive.

Our total capital expenditure has been approximately £70,000.  This does not include insulation, new windows, bathroom items and some labour as these elements were part of the general refurbishment in any event.  Against this we can identify the following financial benefits (using some assumptions)

1. annual saving of £3480  on oil (assuming 5800 litres at 60p per litre)

2. annual revenue of £1783 from Feed-in-Tariff (17,000 kWh at 10.49p per kWh)

3. annual revenue of £432 from exported electricity

4. annual savings of £1495 on electricity we have not had to import (13000 kWhs used out of 20000 generated)

Less expenditure on additional electricity imported at around £2064.  Leaving a positive balance of £5126

From 2014 payment of RHI contributes a further £7743.24 pa for 7 years.  Suggesting a positive balance of £12869 for the seven years post 2014.  Prior to the payment of RHI our estimated payback period was 7-10 years.