Thermal Storage

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MICRO CHP
MICRO CHP
the big picture
By Jeremy Harrison
Photo book

 

Electrical Storage | Thermal Storage | Load Management | Metering

Micro CHP units are unable to provide instantaneous domestic hot water as is common in homes equipped with a combi-boiler, without the addition of a high output supplementary burner.  This is required to meet the very high output, typically between 20-30kWt to achieve a reasonable flow rate.  However, if a large amount of supplementary firing takes place, no electricity is generated and the economic and environmental benefits are reduced; currently only the Remeha Evita product is configured in this fashion, resulting in a significantly reduced economic benefit.

Currently available products, although usually incorporating some supplementary boiler capacity to meet peak demands, are therefore designed to operate in conjunction with a conventional heating system incorporating a hot water cylinder.  Indeed, all low carbon heating systems including solar thermal, heat pumps, biomass boilers and so on, all require some degree of thermal storage to operate effectively.

However, further enhancements in performance can be achieved in conjunction with underfloor heating, whilst even greater benefits can be achieved in conjunction with a primary thermal store (where the stored volume of water is part of the primary heating circuit), which enables the user to run the engine to produce electricity when heat is not required, and to obtain heat when there is no demand for electricity.  This de-coupling of heat and power offers significant benefits in terms of matching power production either to the electrical demand within the home, or to the grid as a whole by matching generation to periods of high market price, which tends also to coincide with high carbon generation from central plant.

 UNDERFLOOR HEATING  THERMAL STORAGE TANK

Underfloor heating has many benefits for home heating regardless of the heat source.  It provides a higher level of comfort for a given air temperature (reducing heat loss and improving energy efficiency) and can make use of lower flow temperatures, enhancing the performance of the heat source. 

In the case of micro CHP the high thermal inertia combined with low flow temperatures can be utilised to improve the efficiency of the engine (Stirling engines are more efficient if the return temperature is lower), although the engine would still need a higher flow temperature for domestic hot water needs.  Effectively, for pipes buried in the concrete floor screed, the mass of the floor above the insulation provides a significant thermal store and reduces cycling of the engine leading to higher efficiency and longer service life.

However, although it is possible to retrofit floor heating providing the householder is prepared to tolerate the disruption and cost, it is much better suited to new-build homes where it can be incorporated optimally during construction.

Although a conventional hot water cylinder does provide a certain level of thermal inertia in the system, a significant improvement can be achieved by use of a dedicated primary thermal store. 

The bulk of the water in the tank forms part of the primary circuit and domestic hot water is produced by passing this water through a high output plate heat exchanger,  effectively the reverse of a standard system.  Alternative configurations pass the cold water through a high efficiency coil running the entire height of the cylinder or in some cases comprise a "tank-in-tank" (as illustrated below).  In order to minimise the bulk of such systems, the inclusion of a quantity of phase change material utilising the high latent heat capacity of, for example, paraffin wax can result in a compact but high capacity unit.

Field trials of this concept with the WhisperGen micro CHP unit have clearly demonstrated the reduction of cycling and longer periods of operation at full output, so that the same level of comfort and higher annual electricity production can be achieved for a given heat load than is possible with higher nominal efficiency micro CHP units without such storage.

For further details contact:

Wirsbo

For further details contact:

Gledhill

Page update 23rd December 2013
 

MICRO CHP
MICRO CHP
the big picture
By Jeremy Harrison
Photo book

 

 

Contact : info@microchap.info

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This site was (partially) last updated on 12th August 2017 Jeremy Harrison