Geothermal Heat Pump Design & Modeling Guide

Good geothermal modeling software has to answer a question about a decade you have not reached yet. A borefield is drilled once, into ground nobody can inspect, and it stays there for the life of the building. Every other piece of equipment can be resized later. The ground loop cannot.

The ground works on two time scales. Over an hour it is a poor heat exchanger: each borehole moves only tens of watts per metre, which is why fields are long. Over a decade it is a huge thermal store that remembers every kilowatt-hour you took out or put in. A field can meet its peak duty for four years and then slowly drift out of range. By the time that shows up, the holes are already in the ground.

The heat pump makes it harder. An air-source heat pump faces the weather, which it cannot change. A ground-source (GSHP) machine faces a loop whose temperature is the running total of everything it has already done. Its efficiency this winter depends on how hard it worked last summer. So capacity, efficiency and even whether the machine may run at all depend on a loop temperature that has to be simulated, not assumed.

Then comes the money. The field size, the heat pump, the backup plant, the tariff and the yearly balance between heat taken from the ground and heat put back all decide whether the project pays back. A sound geothermal techno-economic analysis (TEA) has to treat them as one problem.

The borefield.CogenS models vertical closed-loop fields with GHEtool, an open-source ground-loop engine whose authors validated it against an established commercial design tool in a peer-reviewed paper. It is used unmodified. The model works out the borehole's thermal resistance from the hole size, U-tube layout, pipe, grout and fluid you enter, includes the interference between neighbouring boreholes, and rolls the ground forward year by year across your study period. Every field is checked against the minimum and maximum loop fluid temperatures you set.

The heat pump.Pick a water-source heat pump from the catalogue or enter your own. In the full model, capacity and efficiency (COP) come from the machine's performance curves at each step's loop temperature, with a penalty for cycling at part load. When the loop goes outside your temperature limits, the heat pump stops for that step and the rest of the plant picks up the load. The plant can be a reversible single-circuit machine or a four-pipe plant with two circuits, which changes how much simultaneous heating and cooling can cancel in the ground.

Hybrid balancing. The yearly imbalance between heat put into the ground and heat taken out decides whether a field stays in range. CogenS lets you add the levers that move it: a CHP or boiler for heating, an absorption or electric chiller for cooling, a hybrid loop (cooling tower or dry cooler) to reject surplus heat, storing surplus CHP heat in the ground, and a ground-loop heat pump for domestic hot water. Battery storage and solar can sit on the electric side.

The economics.System Optimization compares up to five equipment combinations over a full year against your utility tariff, including a wet tower's make-up water and any carbon price you set, and recommends the lowest-cost plant within your ground balance target. The full model then replays that plan through the field and equipment you specified, prices it, and builds a TEA report with NPV, IRR, payback, total cost of ownership and a sensitivity analysis.

A common workflow splits the job in two. The loop field is sized in a standalone ground loop design tool from peak and monthly loads, and the economics are built separately in a spreadsheet. The two rarely talk to each other, so the field, the backup plant and the tariff are never traded off against one another.

One model, one plan. In CogenS the optimizer chooses the plant mix and the ground loop together, on the real tariff, and the full model tests that same plan. The report describes the plant the optimizer recommended, and shows in numbers where it fell short once the ground and the heat pump are real: heating and cooling shortfall against the plan, and the share of the planned ground load the field carried.

Balance is a design input. You set how far the ground may end each year out of balance, as a share of the heat moving through it. The optimizer charges any miss so heavily that a balanced design always ranks first, and tells you plainly when no combination can keep the reservoir in balance.

Design for the land you have. Leave the borehole count and depth blank and the model proposes the fewest holes that fit under a practical depth cap, as a recommendation it never types in for you. Enter the field your site can hold and the model measures what that field can carry, designs the plant around it, and shows the no-land-limit field beside it for comparison. It also tests single changes to your field (deeper holes, more holes, wider spacing, double U-tubes, larger pipes, enhanced grout) and reports the capacity each one adds.

Honest answers. If more than 2% of the heating or cooling load goes unserved, the report withholds NPV, IRR and payback rather than show a false return. And if the lowest-cost plant uses no ground loop at all, CogenS says so instead of proposing a token field.