Heat from electricity

The process of generating heat from electricity is expected to have a significant impact in the coming years’ energy supply [22]. This is due to the expected increase in wind power production that will result in an increasing number of hours of excess power production, and thus low electricity prices. Even though the methods for producing heat from electricity have previously been considered less economical due to the general price and tax level for electricity, it allows for a separate production of heat without co-production of electricity.

Another reason is the uncertain future for biomass fueled CHP production, especially if biomass become a scarce resource for sustainable heat and power production.

Two known methods to produce heat from electricity using an EB and a HP. Both have different advantages and disadvantages, potentially making them suitable in different situ-ations. The following will give a brief overview of the two methods, including the mutual differences and the integration potential.

Electric heat boiler

The EB is a simple technology that converts electrical power into thermal power with an efficiency of approximately 1. The principle is illustrated in Figure 2.5(b) and the corresponding electrical diagram is shown in Figure 2.5(a).

EBs have the advantage of being very flexible. The unit is capable of starting up in a few seconds and up and down regulate the production with similar speed only with marginal loses in efficiency. No fuel feeding system or stack is required as electricity is the only source.

Furthermore, EBs are based on a well developed and tested technology involving no complex components [23]. This makes it extremely reliable and easy to maintain. Already existing EBs typically have capacities spanning 1-25 MW, while larger capacities are obtained by coupling of units. EBs are commercially available and they are considered a cheap invest-ment with prices around 0.15 mio AC per MW for small EBs and decreasing unit costs for larger EBs [23, 24]. However, EBs have the disadvantage of being completely dependent on electricity and thus the electricity prices. The operational costs therefore vary with the variable electricity prices which together wit taxes generally has been too high for the EB

2.1 Heat and power production 11

V Heater

Electricity

A

(a)

District heating

Return water

Heater

Electricity

(b)

Figure 2.5 – (a) Circuit diagram for an EB. (b) Illustrative example of an EB providing heat for district heating.

to be very profitable.

Heat pump

Heat flows naturally from a higher to a lower temperature. However, HPs are able to force the heat flow in the other direction, using a relatively small amount of drive energy such as electricity, fuel, or high-temperature waste heat. The focus will here be on electricity driven HPs.

The principle of a HP is identical to that of a reverse refrigerator. For HPs, the heat that is extracted from the ”refrigerator” is the interesting part. Figure 2.6 illustrates the working principle.

Cold heat source Compressor Expansion

valve

Condenser

Evaporator

0 ºC 10 ºC

85 ºC 40 ºC

Heating network

Figure 2.6– Diagram of a HP. In the compressor the temperature of the refrigerant is increased by compression which is subsequently exhanged with water to be heated in the condenser. An expansion valve descrease the pressure and the cycle continues.

Energy from the cold source is transported to the heating network by a refrigerant, which has specific thermodynamic properties. At the evaporator the refrigerant absorbs heat and vaporizes. Subsequently, the refrigerant is compressed to increase the temperature. The compressor is driven by an electrical motor which is the main part to consume electricity.

In the condenser the refrigerant is cooled such that it condenses and release heat to the heating network (district heating). Finally, the expansion valve lowers the pressure and the cycle starts again.

Several options exists for the cold heat source: Air, sea water, waste water and geothermal energy are examples of some of the most frequently used. The choice of cold heat source reflects the stability and performance of the HP. If air is used, and the air temperature varies significantly during the year, the performance will vary accordingly and possibly lead to an unstable system [24]. This argues for use of geothermal heat, sea water or waste water as less variation is found for these sources.

The most commonly applied refrigerant is currently ammonia (NH₃). However CO₂ is also starting to be applied due to superior abilities to extract heat from cold sources below ≈ 20^◦C and its ability to provide high condensing temperatures.

The efficiency of the HP varies depending on the temperature requirements. The coefficient of performance (COP) describe the ratio between heat output and electricity input. The theoretical COP for a HP is calculated based on the Carnot efficiency [25]:

COPcarnot= T_h T_h−T_l

where Th is the supply temperature and Tl is the temperature of the cold medium both in K. If a waste water temperature of approximately 10^◦C (283 K) and an output water temperature of 85^◦C (358 K) is assumed, the resulting Carnot COP is:

COP_Carnot= 358K

358K−283K = 4.8

If geothermal water is used instead, the cold medium temperature would be around 50^◦C (323 K) [18], resulting in a much higher efficiency of:

COP_Carnot= 358K

358K−323K = 10.2

It should be emphasized that these are theoretical maximum efficiencies. In reality it has been found that the efficiency is approximately 50-70% of the Carnot efficiency [18]. The COP for HPs are therefore in reality typically between 2 and 5, even though higher values can be obtained. In addition to the temperature, several other factors such as the compressor efficiency and choice of refrigerants also affects the COP.

Just as the EB, the HP is a flexible solution for separating heat and power production. It has a high efficiency and is comparably less dependent on the electricity price. The HP can utilize heat from otherwise wasted sources such as waste water, sea water or geothermal heat. However, due to the complex structure of HPs they require extensive investments with long pay back times. The prices are approximately 0.5 mio AC per MW output, and furthermore, maintenance costs should also included [23].

Compared to the EB, the HP is not as flexible in terms of ramping during start-up and shut-down. Figure 2.7 illustrates this issue simply. For the EB, start-up occurs almost