Heat pumps are the main contender for hydrogen as a means to decarbonise the heating of domestic and commercial premises. Hydrogen can be produced by steam reforming natural gas and is free of most greenhouse gas emissions if CCS is applied. Because of the high cost and inflexibility of steam reforming installations it is essential to have high pressure underground storage of hydrogen to smooth out seasonal variations in supply. Reforming, CCS and high pressure storage all consume energy. Thus the energy content of hydrogen supplied via this route is considerably less than in the natural gas feedstock. Taking account of the capital and operating costs of the equipment the cost of bulk hydrogen supply is likely to be at least double that of natural gas. However distribution costs greatly exceed bulk supply costs so consumers would see their costs increase by a much smaller factor. 

On the positive side, existing gas distribution networks have the capacity to distribute hydrogen with only minor modifications. Existing designs of gas boilers and cookers can easily be modified to operate on hydrogen and most can be converted in a similar way to that used when town’s gas was replaced by North Sea gas in the UK. 

 

Heat pumps use electricity to raise the temperature of a low grade heat source. Typically one unit of electricity can provide 3 or 4 units of heat at a temperature suitable for heating buildings. They are less suitable for providing hot water and have no potential for cooking

. 

The cost of electric power delivered to consumers is typically 3 to 4 times that of energy supplied as gas. The energy costs for heating by heat pump is thus similar to that for heating by gas or fossil fuel derived from hydrogen as these two factors cancel out.  

If fossil fuels are phased out hydrogen could be produced by electrolysis of water with electricity. Some energy is lost in this process meaning that from a pure efficiency point of view it might be better to consume electricity directly rather than convert it to hydrogen. This leaves hydrogen with only one key advantage, the ability to store large quantities in underground caverns or reservoirs. 

 

Heat pumps might appear to have a considerable advantage once hydrogen has to be manufactured using electric power. However they have a number of disadvantages. These depend on the source of low grade heat which they use. The most readily available is air. This can be drawn through the evaporator of the heat pump using a fan. Such units are placed outside the premises to be heated either on an external wall or a roof. In densely populated areas finding suitable locations can be difficult especially if maintenance access and neighbourhood noise is taken into account. 

An alternative is to use ground or flowing water as a heat source. In all cases the primary source of the low grade heat is the sun. This heats the air used on air source units, it heats flowing water directly or indirectly by heating the ground. Where solid ground is used, usually by burying heat exchange coils in shallow trenches, this is replenished up to a depth of a few meters by solar heating. If heat is withdrawn from deeper wells all of the heat withdrawn in winter has to be replenished by pumping hot fluid down in the summer. For a zero carbon system this hot fluid would inevitably have to be heated by renewable energy. This can be by using waste heat from summer air-conditioning or collection of solar thermal energy. It is a myth that geothermal heat rising from the earth’s core can be used as the heat flow of around 65 milliwatts per m² is orders of magnitude too low. Below a few meters depth reheating by conduction of incident solar radiation is also far too low.

 

Cost is the other main disadvantage of heat pump technology. The units themselves are several times the cost of a conventional boiler. In addition alternative provisions have to be made for cooking and hot water. In the absence of gas that means direct electric heating although part of the hot water provision can be by solar thermal. A further issue is the temperature at which heat is delivered in properties. Gas or oil heated premises usually need water at up to 70C which circulates through radiators even though the target temperature for the rooms is only around 20C. This temperature is too high for heat pumps to be efficient. Air source heat pumps get round this by supplying warm air which is blown through the unit with a fan. Where hot water based systems are used the lowest circulating temperatures can be achieved by underfloor heating, otherwise oversized radiators are used. Operation is continuous supplying only the temperature required to reach target whereas gas boilers usually work in on off mode to supply a target circulating water supply temperature. Such a mode of operation would be very inefficient for a heat pump.  

Even though a heat pump based system would use far less renewable energy than one based around hydrogen the grid supply would have to significantly upgraded as it is not usually designed for the domestic/commercial heating load to be electric. Individual properties are likely to have adequate capacity but the local supply networks are most likely to need upgrade to cope with the extra demand. 

Technologies which are able to remove some of these barriers would greatly help to enhance the uptake of the more energy efficient heat pump technology. I have been investigating what possibilities exist and am currently exploring two options which might be used. 

The first is a two stage air source system in which the first stage is centralised for a street or group of streets. The heat is distributed by a district heating system but users have their own second stage heat pump and can raise the heat level to whatever temperature suits their property. This minimises heat losses in the distribution system, allows the frost prone part of the system to be operated centrally and removes the noise of the air units to just one less intrusive location. I have published details of this concept in the May 2021 issue of The Chemical Engineer.

The second approach is to use a set of centralised deep wells for heat supply. I am modelling a system in which single central well with a 200-300m long heated section is surrounded by a ring of 8 heat supply wells about 5 to 10 meters away. These effectively encase the central well which is heated to very high temperatures, possibly as high as 200-300C, by concentrated solar thermal energy. Initial simulations suggest that such a system could satisfy the demands of around 100 moderate sized properties and possibly far more smaller and better insulated homes. Thermodynamically this system is rather inefficient as high grade heat is collected and degraded when it is stored in the underground formation. However since the only use for this heat would be for heating properties this inefficiency may not matter. Some could no doubt be used for hot water heating or power generation without too much complication.  

To generate high temperature solar thermal heat a dedicated array of considerable area would be needed. Such a system could be feasible outside small towns but would be less appropriate in larger conurbations. 

I plan to publish some results in due course.