Renewable heat

From Energy-Experts

Claverton Renewable Heat: Tuesday 22nd March 2008

Introduction

The UK Government has been asking for suggestions on how Britain’s need for heating will be met once fossil fuel runs out or becomes uncompetitive as an energy source. The Government is to be applauded for focusing on this important question. Up to the present time virtually all of the Government’s attention has been on electricity generation, and it is good to see other issues being reviewed.

This note is an attempt to summarise the various options that are possible in terms of renewable heat. However, since a major source of renewable heat is renewable electricity from wind, solar, wave and tidal resources, it becomes easy to confuse some of the suggestions as being those related to the storage of renewable electricity. It is vitally important not to confuse the two issues. Although stored heat, in terms of hot water or ceramic type storage heaters, does represent a means of storing electricity, it is not the purpose of this note to suggest this as being the best means of storing spare renewable electricity.

In this note, no economic estimates are given for each particular option. Hopefully the proponents of each system will provide figures which are reasonably objective. The only points which should be kept in mind are that the housing structure in the UK will evolve only slowly, and heating requirements are not likely to change very radically. The best that can be hoped for is a gradual improvement in insulation standards in the existing housing stock, and that new buildings will begin to confirm to the requirements and spirit of the EU Buildings Directive.

Space Heating in the Domestic and Commercial Sectors

The main need for energy in the UK, excluding transport is for space heating in the domestic and commercial sectors. It makes economic sense to utilise as much of the existing heating systems as possible, in the utilisation of renewable heat.

At the present time, the main source of energy which is used for space heating is usually natural gas, although there is limited use of electricity and fuel oil. The use of fuel oil seems to be confined to rural areas, and because of this there is some interest in the use of locally grown biomass as an alternative.

Where gas and oil are used, the fuels are burnt and the heat utilised for water-type central heating. Assuming that both gas and oil disappear from the space heating market, either because of concern about greenhouse warming, diversion of these fuels into the transport market, or the exhaustion of fossil fuel reserves, this leaves a number of different options for the use of renewable energy sources. These options are:

• Replace gas or fuel oil directly in central heating systems. If so the possible options are solar heating, hydrogen, dried wood, liquid biofuels, or, in certain circumstances, wood or charcoal type products

• Use renewable electricity as a source of energy to heat up the water in central heating systems

• Use renewable electricity to drive heat pumps which could heat up water in central heating systems

• Establish CHP or District Heating Systems which will utilise biomass, hydrogen or renewable electricity as the energy source

• Use renewable electricity in storage heaters

• Use renewable electricity directly, using warm air or radiant (resistance-type) heaters

It will be noted that the use of the direct use of renewable electricity is put last in this series of options. In the UK, most of the renewable electricity will come from wind power. And since this is an intermittent and an irregular source of electricity, wind energy needs to be backed by some form of energy storage system. This will not only to cater for windless periods when there is no wind energy, but to absorb wind energy produced in excess of the amount actually needed. The use of resistance heaters puts a tremendous near-instantaneous demand on the grid and was largely responsible for the blackouts in the winter of 1962-63.

Related to this subject is that there may be problems because of the limitations of the local grid. At the present time this is limited to about 2kW per household, the critical point being the 11kV/440K transformers and the wiring that comes from these. The wiring into individual households is not likely to impose a limit in terms of current carrying capacity.

However the local grid limit is far below what is possible with the gas supply. Any proposal which requires a house-by-house increase in the electricity demand, for example heat pumps and storage heaters, needs to critically examine whether the local electricity distribution system will need strengthening.

All of the other systems have an energy storage aspect, although this may have to be supplemented by the use of extra hot water cylinders. But the option of replacing natural gas with hydrogen, which leaves the space heating market as it is today, is somewhat different from the alternatives in that hydrogen can be used as an energy store.

None of the proposed solutions is perfect. All have shortcomings, but it is a question of identifying into which particular niche best fits each of these ideas. Some of the main points are summarised below.

Solar Heating of Water: This is an important source of heat, even in the UK. The main disadvantage is that it shuts down completely in snowy weather, and so it best combined with other forms of renewable heating

Hydrogen: The only viable source of hydrogen is renewable electricity which is used to electrolyse water. Unfortunately there are losses of about 30% in using electricity in this process. There are further losses of about 10-20% in using the hydrogen as a heat source in the central heating system, although condensing boilers may be beneficial. The result is that hydrogen, if costed on the same basis as electricity, will need to be priced at about 1.5-2.0 times the price of electricity. The main advantage of hydrogen is that it is possible to store it indefinitely and the existing natural gas pipeline and gasholder network can be used for this purpose. It could also be stored in exhausted natural gas or oil production reservoirs.

Furthermore if the electricity from wind, is only used for hydrogen production, when there is no other use, it can be regarded as being “free”. In practice some payment would be required but this would be at a price the market would bear. In this way hydrogen would be more competitive. However, it also follows that the gas-based central heating market will shrink significantly.

Biofuels: The availability of these will be limited, with most going into the transport market. If these are available their use should be limited to local CHP systems

Biomass: Experience in Austria suggests that the best use of these seems to be in small to medium sized district heating systems, using small pellet boilers

Electricity as an Energy Source in Central Heating Systems: Here immersion type heaters replace the gas boiler. The main problem is that the power demand can be very high during the early morning start up of central heating systems. This can be mitigated to some extent by an increase in the hot water storage capacity and increasing the hot water storage temperature to higher temperatures.

Heat Pumps: The cost of the heat pump itself is one issue, but hopefully with mass production, this should not be too much higher than a gas boiler. The biggest problem is the heat source. Using the earth (i.e. gardens) as heat pump reservoir will be unacceptable to most people on economic and amenity grounds. In some parts of the UK, air will be practical and cheap. However a good case can be made for using the water in a district heating system as the source, since this will improve the efficiency of the district heating boiler plant.

District Heating using Renewable Electricity or Biomass: The main advantage of such an approach is that the district heating system acts as a heat reservoir. Furthermore it is possible to store heat for considerable periods in insulated pressure vessels. The disadvantage is the need, in the UK, to put in a completely new energy network. It should be considered, however, as a priority, for new housing and commercial developments

Storage Heaters: Although a viable option, if it is possible to guarantee the overnight supply of electricity at a high level, storage heaters will have problems if there is a shortage of power for several days. Nevertheless, with increased insulation standards, and the tendency to people living in small houses and apartments, storage heaters could become one of the best options.

F.Starr/D.Andrews on behalf of the Claverton Group