Greenhouse sites as heat providers
Essentially, any site with a heat demand provides a potential connection to a heat network.
A nearby growing site, other commercial heat users, or local domestic properties will have different requirements for temperatures, heating profiles, and available capacity. Understanding these is key to designing the distribution pumping equipment, heat transfer equipment, and standby top-up heating arrangements.
An ideal site for integration into a CHP-based horticultural heat network is one that can take low-grade heat. Often useful heat for a greenhouse site is high-grade, around 80⁰C. If lower grade heat from the same CHP unit (which is typically dumped), at 60⁰C or even 40⁰C can be used, this will increase the efficiency of the generator by making use of this ‘free’ heat. Examples of low-temperature heat emitters are underfloor heating and fan coils.
As a heat provider, sizing considerations come in two forms: heat generating size, and distribution pipe size. The generating capacity should be sized based on the number of users you want to have on the network, their relative heat demands and heat schedule. For example, If two sites never need heat at the same time, there is no need to combine their heat demands.
Distribution pipework will mainly depend on the temperature differential at which you are running the system, and the pressure required to transport the water throughout your network. The specifics on pipe sizing are discussed in more detail in the ‘Pumps and pipes’ section
The most efficient way to run a heat network is to have a large difference in flow and return temperatures. The larger the difference between flow and return temperatures, the more heat that can be delivered with the same amount of water, effectively reducing pumping costs per kWh of delivered heat, as well as decreasing fuel costs by making use out of more of the generated heat. Heat differentials between flow and return of 20⁰C to 30⁰C are a good point to aim at.
An important component to delivering heat consistently is accumulator tanks, which store hot water for future use and can be placed either at the point of generation or at the use. Domestic properties may prefer small accumulator tanks so that heat and hot water are available instantly, whereas a commercial user may be willing to wait for several minutes+ before heat is delivered. Accumulator tanks also allow for the different heating profiles across the network; by storing excess heat at the time of generation they reduce the run-hours of the heating asset, which can increase lifetime.
Another benefit is that if heat is generated out of synchronisation with the use, it can be stored until it is needed, for example, when using a heat pump, it can produce heat during non-peak electricity consumption hours, and stored. This provides a cheaper running cost and grid balancing benefits.
What happens when things go wrong?
You should consider what to do during times of planned maintenance, or when your heat generator stops working for whatever reason. This could be in the form of provision of a backup boiler, which can deliver a minimum contracted heat. Inevitably, the system will fail at some point, and the contracts should be written to specify what happens during those periods.